Biomarker Consortium

Breast Cancer

Triple-Negative (TNBC)

); encodes programmed death-ligand 1 (PD-L1), also designated B7-H1

PD-L1 is a transmembrane immune-checkpoint ligand expressed on tumor cells and on tumor-infiltrating immune cells. Binding to PD-1 on activated T cells delivers an inhibitory signal that blunts cytotoxic T-cell activity.

Under normal conditions this axis restrains T-cell responses to limit collateral tissue injury and maintain peripheral tolerance. Tumors co-opt it: PD-L1 is upregulated in the breast tumor microenvironment largely through interferon-gamma released by infiltrating lymphocytes, a mechanism termed adaptive immune resistance.

PD-1 blockade releases that brake and restores T-cell effector function. In advanced breast cancer the benefit has been demonstrable only in tumors with high PD-L1 expression; in high-risk early-stage TNBC the perioperative benefit was seen irrespective of PD-L1 status.

Unlike a genomic alteration, PD-L1 expression is a dynamic phenotypic marker of the tumor immune microenvironment. It can differ between the primary tumor and a metastatic site and can change after intervening therapy.

Triple-negative breast cancer accounts for roughly 15% of breast cancers and carries the highest tumor-infiltrating lymphocyte burden and the highest PD-L1 positivity of the major breast subtypes.

In KEYNOTE-355, approximately 38% of previously untreated locally recurrent inoperable or metastatic TNBC tumors had a combined positive score (CPS) of 10 or higher, and approximately 75% had CPS of 1 or higher, by the 22C3 assay.

A CPS of 10 or higher is currently the only PD-L1 threshold that gates a breast cancer treatment decision in the United States. It governs both the first-line pembrolizumab plus chemotherapy indication and the June 2026 sacituzumab govitecan plus pembrolizumab indication.

Test at the diagnosis of locally recurrent unresectable or metastatic TNBC, before first-line systemic therapy is selected, because the result determines eligibility for pembrolizumab plus chemotherapy and for sacituzumab govitecan plus pembrolizumab.

PD-L1 testing is not required for the high-risk early-stage TNBC perioperative indication, which is unselected for PD-L1. A metastatic-setting result is still required later if checkpoint therapy is considered at recurrence.

Because expression can shift after intervening therapy and differs across sites, test the most recent adequate tumor specimen where feasible rather than relying on an archival primary alone.

Immunohistochemistry on formalin-fixed, paraffin-embedded tumor tissue is the only validated approach; there is no plasma or circulating tumor DNA assay for PD-L1.

CPS is the number of PD-L1-staining cells (tumor cells, lymphocytes, and macrophages) divided by the total number of viable tumor cells, multiplied by 100.

PD-L1 IHC 22C3 pharmDx (Agilent/Dako) is the companion diagnostic used to define CPS of 10 or higher in both KEYNOTE-355 and ASCENT-04/KEYNOTE-D19. It was approved concurrently with the November 2020 pembrolizumab accelerated approval.

The Ventana PD-L1 (SP142) assay scores immune-cell staining rather than CPS and is not interchangeable with 22C3. It is no longer tied to an active US breast cancer indication following withdrawal of the atezolizumab metastatic TNBC indication in 2021.

ASCO recommends PD-L1 assessment by a validated CPS assay in patients with metastatic TNBC being considered for first-line checkpoint-inhibitor-based therapy.

NCCN includes PD-L1 testing by an FDA-approved assay in the workup of recurrent or stage IV TNBC, with CPS of 10 or higher directing patients toward a pembrolizumab-containing first-line regimen.

One agent is FDA-approved in breast cancer with PD-L1 CPS as its selection biomarker: pembrolizumab (Keytruda), available intravenously and as a subcutaneous coformulation with berahyaluronidase alfa-pmph (Keytruda Qlex).

On November 13, 2020, the FDA granted accelerated approval to pembrolizumab plus chemotherapy for locally recurrent unresectable or metastatic TNBC with CPS of 10 or higher, based on the phase 3 KEYNOTE-355 trial, and approved PD-L1 IHC 22C3 pharmDx as the companion diagnostic. On July 26, 2021, the FDA approved pembrolizumab for high-risk early-stage TNBC based on the phase 3 KEYNOTE-522 trial and, in the same action, 

converted the metastatic accelerated approval to regular approval

 on the basis of confirmatory KEYNOTE-522 data.

Since June 24, 2026, a CPS of 10 or higher additionally selects patients for sacituzumab govitecan plus pembrolizumab in the first-line metastatic setting, based on ASCENT-04/KEYNOTE-D19. That regimen is covered on the TROP2 biomarker page.

In combination with chemotherapy, for adults with locally recurrent unresectable or metastatic triple-negative breast cancer whose tumors express PD-L1 (CPS greater than or equal to 10) as determined by an FDA-authorized test.

For adults with high-risk early-stage TNBC, in combination with chemotherapy as neoadjuvant treatment, then continued as a single agent as adjuvant treatment after surgery.

Humanized IgG4 monoclonal antibody that binds PD-1 and blocks its interaction with PD-L1 and PD-L2.

Blockade relieves checkpoint-mediated inhibition of tumor-specific T cells and restores antitumor immune response.

Agents No Longer Indicated in Breast Cancer

Atezolizumab plus nab-paclitaxel held accelerated approval for PD-L1-positive (SP142 immune-cell scoring) metastatic TNBC from March 2019. The indication was voluntarily withdrawn in the United States in 2021 after IMpassion131 failed to confirm benefit. Retain only as historical context; it is not a current treatment option.

1. KEYTRUDA (pembrolizumab) injection. Prescribing information. Merck Sharp & Dohme LLC. Accessed July 2026.

2. Cortes J, Rugo HS, Cescon DW, et al. Pembrolizumab plus chemotherapy in advanced triple-negative breast cancer (KEYNOTE-355). 

. 2022;387(3):217-226. doi:10.1056/NEJMoa2202809

3. Schmid P, Cortes J, Dent R, et al. Overall survival with pembrolizumab in early-stage triple-negative breast cancer (KEYNOTE-522). 

. 2024;391(21):1981-1991. doi:10.1056/NEJMoa2409932

4. US Food and Drug Administration. FDA approves sacituzumab govitecan-hziy as monotherapy and in combination with pembrolizumab for first-line treatment of triple-negative breast cancer. June 24, 2026. Accessed July 2026.

5. PD-L1 IHC 22C3 pharmDx. Instructions for use. Agilent Technologies/Dako North America, Inc. Accessed July 2026.

6. US Food and Drug Administration. FDA grants accelerated approval to pembrolizumab for locally recurrent unresectable or metastatic triple negative breast cancer. November 13, 2020. Accessed July 2026.

7. Genentech/Roche. Voluntary withdrawal of the atezolizumab accelerated approval for PD-L1-positive metastatic triple-negative breast cancer in the United States. 2021. Accessed July 2026.

8. ASCO Guideline: Biomarkers for Systemic Therapy in Metastatic Breast Cancer. Accessed July 2026.

9. NCCN Clinical Practice Guidelines in Oncology (NCCN Guidelines®) for Breast Cancer V.5.2026. © National Comprehensive Cancer Network, Inc. 2026. All rights reserved. Accessed July 2026. To view the most recent and complete version of the guideline, go online to NCCN.org.

10. Merck & Co, Inc. FDA approves KEYTRUDA (pembrolizumab) for treatment of patients with high-risk early-stage triple-negative breast cancer. News release. July 26, 2021. Accessed July 2026.

PD-L1

); encode tumor suppressor proteins central to homologous recombination repair (HRR) of DNA double-strand breaks

Gene Location: Chromosome 17 (17q21.31) for 

In an HRR-proficient cell, BRCA1 coordinates cell-cycle checkpoint control and DNA end resection, while BRCA2 loads RAD51 onto resected single-stranded DNA, enabling error-free repair of double-strand breaks.

Tumorigenesis follows a two-hit pattern: an inherited loss-of-function allele plus somatic loss of the remaining wild-type allele leaves the tumor, but not normal tissue, HRR-deficient. Cells are forced onto error-prone backup repair pathways, producing the genomic instability that creates the therapeutic window.

PARP inhibitors block PARP1 catalytic activity and trap PARP1 on DNA at single-strand break sites. The trapped complexes collapse replication forks into double-strand breaks that an HRR-deficient tumor cannot repair, producing synthetic lethality. The same deficiency underlies sensitivity to platinum chemotherapy.

 pathogenic variants are inherited and present from birth, not acquired under treatment pressure. Status is established once and does not require retesting at progression, in contrast to acquired markers such as 

-associated tumors are predominantly triple-negative and basal-like, whereas 

-associated tumors are more often hormone receptor-positive. Carriers are enriched among patients with young-onset disease, bilateral disease, and a suggestive family history.

Triple-negative breast cancer carries the highest carrier frequency of the major subtypes.

 from blood or saliva. Tumor-only next-generation sequencing cannot distinguish germline from somatic origin and does not establish eligibility for the germline-restricted PARP inhibitor indications; a 

 finding on a tumor panel requires germline confirmation.

 testing to all patients diagnosed with breast cancer at or below age 65. Above age 65, offer testing to patients who are candidates for PARP inhibitor therapy, have triple-negative disease, were assigned male sex at birth, or have a personal or family history suggestive of a pathogenic variant.

Offer testing regardless of age to any patient with local or metastatic recurrent breast cancer who is a candidate for PARP inhibitor therapy, and to patients with a second primary in the contralateral or ipsilateral breast.

Multigene panel next-generation sequencing on germline DNA, combining sequence analysis with large-rearrangement and copy-number analysis. Deletion and duplication testing matters, since large rearrangements are a meaningful minority of 

 pathogenic variants and are missed by sequencing alone.

BRACAnalysis CDx (Myriad Genetic Laboratories) is the FDA-approved companion diagnostic for both olaparib and talazoparib in breast cancer, covering the metastatic indications and the adjuvant olaparib indication.

Panels typically also report the high-penetrance genes 

. Moderate-penetrance findings inform second-primary and family risk but do not create an approved treatment option in breast cancer.

Variants of uncertain significance are common and are not actionable. A VUS should not be treated as a positive result in therapy selection or in risk-reducing surgical planning.

ASCO and the Society of Surgical Oncology published a joint germline testing guideline for breast cancer in 2024, which sets the age-65 threshold and directs additional predisposition-gene testing based on personal and family history.

 status in the workup of recurrent or stage IV HER2-negative disease and of high-risk early-stage disease where adjuvant olaparib is under consideration, with detailed testing criteria in the NCCN Guidelines for Genetic/Familial High-Risk Assessment: Breast, Ovarian, Pancreatic, and Prostate.

Two oral PARP inhibitors are FDA-approved specifically for germline 

-mutated, HER2-negative breast cancer: olaparib (Lynparza) and talazoparib (Talzenna). Both require selection by an FDA-approved companion diagnostic performed on germline DNA.

On January 12, 2018, the FDA approved olaparib for germline 

-mutated, HER2-negative metastatic breast cancer based on the phase 3 OlympiAD trial, which demonstrated improved progression-free survival versus physician's choice chemotherapy. On March 11, 2022, the indication was extended to the adjuvant treatment of high-risk early breast cancer based on the phase 3 OlympiA trial.

On October 16, 2018, the FDA approved talazoparib for germline 

-mutated, HER2-negative locally advanced or metastatic breast cancer based on the phase 3 EMBRACA trial. Talazoparib has no adjuvant breast cancer indication.

Olaparib is the only PARP inhibitor with an early breast cancer indication, and the only one to have demonstrated an overall survival benefit in breast cancer, in the adjuvant OlympiA setting.

Adults with deleterious or suspected deleterious germline BRCA-mutated, HER2-negative metastatic breast cancer previously treated with chemotherapy in the neoadjuvant, adjuvant, or metastatic setting. Patients with hormone receptor-positive disease should have received prior endocrine therapy or be considered inappropriate for it.

Adjuvant treatment of adults with deleterious or suspected deleterious germline BRCA-mutated, HER2-negative high-risk early breast cancer previously treated with neoadjuvant or adjuvant chemotherapy.

Oral inhibitor of PARP1, PARP2, and PARP3 that blocks catalytic activity and traps PARP at sites of DNA damage.

Trapped PARP-DNA complexes collapse replication forks into double-strand breaks that HRR-deficient tumor cells cannot repair, producing synthetic lethality.

Adults with deleterious or suspected deleterious germline BRCA-mutated (gBRCAm), HER2-negative locally advanced or metastatic breast cancer. Patients must be selected based on an FDA-approved companion diagnostic.

Oral PARP inhibitor with potent PARP-trapping activity, blocking PARP1 and PARP2 catalytic function and stabilizing PARP-DNA complexes.

The resulting replication-associated double-strand breaks cannot be repaired in HRR-deficient tumor cells.

HRR deficiency also predicts platinum sensitivity, and platinum agents remain a common comparator and sequencing consideration in germline 

-mutated disease. They are not biomarker-gated by label.

 germline pathogenic variants produce a comparable HRR-deficient phenotype, and small single-arm studies have shown high response rates to PARP inhibition. There is currently no FDA-approved PARP inhibitor indication for 

 in breast cancer; use in that setting is off-label.

1. Roy R, Chun J, Powell SN. BRCA1 and BRCA2: different roles in a common pathway of genome protection. 

2. BRCA1 and BRCA2: Cancer Risks and Management (PDQ). National Cancer Institute. Accessed July 2026.

3. LYNPARZA (olaparib) tablets. Prescribing information. AstraZeneca Pharmaceuticals LP. Accessed July 2026.

4. TALZENNA (talazoparib) capsules. Prescribing information. Pfizer Inc. Accessed July 2026.

5. Merck & Co, Inc. LYNPARZA (olaparib) reduced risk of death by 32% in patients with germline BRCA-mutated, HER2-negative high-risk early breast cancer in the phase 3 OlympiA trial. News release. Accessed July 2026.

6. Bedrosian I, Somerfield MR, Achatz MI, et al. Germline testing in patients with breast cancer: ASCO-Society of Surgical Oncology guideline. 

. 2024;42(5):584-604. doi:10.1200/JCO.23.02225

7. NCCN Clinical Practice Guidelines in Oncology (NCCN Guidelines®) for Breast Cancer V.5.2026, and for Genetic/Familial High-Risk Assessment: Breast, Ovarian, Pancreatic, and Prostate V.3.2026. © National Comprehensive Cancer Network, Inc. 2026. All rights reserved. Accessed July 2026. To view the most recent and complete version of the guideline, go online to NCCN.org.

8. US Food and Drug Administration. FDA approves olaparib for germline BRCA-mutated metastatic breast cancer. January 12, 2018. Accessed July 2026.

9. US Food and Drug Administration. FDA approves olaparib for adjuvant treatment of high-risk early breast cancer. March 11, 2022. Accessed July 2026.

10. US Food and Drug Administration. FDA approves talazoparib for gBRCAm HER2-negative locally advanced or metastatic breast cancer. October 16, 2018. Accessed July 2026.

BRCA1/2

Tumor-associated calcium signal transducer 2 (

); encodes trophoblast cell-surface antigen 2 (Trop-2), a 40-kDa transmembrane glycoprotein of the EpCAM family

Trop-2 is a calcium signal transducer involved in epithelial proliferation, adhesion, and migration. It is expressed at low levels in normal epithelium and upregulated across essentially all epithelial carcinomas, including every breast cancer subtype.

Upregulation has been linked to accelerated tumor growth and less favorable prognosis, with signaling crosstalk through the PI3K/AKT/mTOR, MAPK/ERK, JAK/STAT, and Wnt/beta-catenin pathways.

, Trop-2 is not a therapeutic driver. It functions as a 

: an antibody-drug conjugate binds surface Trop-2, internalizes, and releases a topoisomerase I inhibitor payload (SN-38 for sacituzumab govitecan, DXd for datopotamab deruxtecan). Both payloads are membrane-permeable enough to produce bystander killing of adjacent tumor cells.

Trop-2 expression is near-universal in breast cancer and present at diagnosis rather than acquired under treatment pressure. In one 77-patient surgical series, 96.1% of breast tumors were Trop-2-positive, including 97.1% of triple-negative and 93.5% of HR-positive/HER2-negative tumors.

Because expression is close to universal, 

no cut point has been clinically validated

 that separates responders from non-responders to the approved TROP2-directed agents. Expression level is not used to select patients.

Marked intratumoral heterogeneity and discordance between primary and metastatic sites further complicate any threshold-based approach, and membranous versus cytoplasmic localization appears to matter more than total staining intensity.

 before either approved TROP2-directed antibody-drug conjugate. Neither sacituzumab govitecan nor datopotamab deruxtecan carries an FDA-required companion diagnostic for TROP2 in breast cancer.

The tests that actually gate these agents define the clinical setting: HR and HER2 status, including the IHC 0, IHC 1+, and IHC 2+/ISH-negative categories used in the HR-positive indications; 

 to establish PD-1/PD-L1 candidacy in first-line TNBC; and germline 

 status where a PARP inhibitor is the competing option.

Immunohistochemistry on formalin-fixed, paraffin-embedded tissue remains the primary method of assessing Trop-2 protein. In investigational settings, moderate-to-strong membranous staining in at least 10% of tumor cells has commonly been used to designate TROP2-high; by that convention 82% of triple-negative tumors qualified in one 807-patient cohort.

Quantitative continuous scoring (QCS) applies a deep-learning digital-pathology algorithm to whole-slide images to derive a normalized membrane ratio (NMR), quantifying membrane-bound Trop-2 relative to total Trop-2. In the TROPION-Lung01 exploratory analysis, tumors were called QCS-NMR positive when at least 75% of tumor cells had an NMR at or below 0.56, indicating a greater cytoplasmic proportion; 63% of biomarker-evaluable patients were positive. QCS-NMR-positive patients treated with datopotamab deruxtecan had an objective response rate of 32.7% versus 10.3% with docetaxel and median progression-free survival of 6.9 versus 4.1 months (HR 0.57), with no separation in the QCS-NMR-negative group.

QCS-NMR was developed and tested in non-small cell lung cancer, not breast cancer. It remains investigational, is not standardized across laboratories, and has no role in breast cancer treatment selection today. Note also that the positivity direction is counterintuitive: a 

 membrane ratio defines the favorable group.

There is no companion diagnostic for TROP2 and no validated plasma or circulating tumor DNA assay for TROP2 expression. For the sacituzumab govitecan plus pembrolizumab indication, patient selection is by PD-L1 IHC 22C3 pharmDx rather than by any TROP2 assay.

UGT1A1 status is relevant to sacituzumab govitecan safety, since reduced UGT1A1 activity raises the risk of neutropenia, but the label does not mandate testing before treatment.

No major guideline body currently recommends routine TROP2 testing in breast cancer, and ASCO biomarker guidance does not list TROP2 among biomarkers required to direct systemic therapy.

NCCN positions the TROP2-directed agents by clinical setting, subtype, and prior therapy rather than by a TROP2 result. Sacituzumab govitecan is listed as a preferred category 1 first-line option for metastatic TNBC with PD-L1 CPS below 10 and no germline 

Two TROP2-directed antibody-drug conjugates are FDA-approved in breast cancer: sacituzumab govitecan (2020) and datopotamab deruxtecan (2025). Neither is selected by a TROP2 assay.

Sacituzumab govitecan was first approved in April 2020 for pretreated metastatic TNBC, with regular approval in April 2021 based on ASCENT, expansion to HR-positive/HER2-negative disease in February 2023 based on TROPiCS-02, and two first-line TNBC indications on June 24, 2026 based on ASCENT-03 and ASCENT-04/KEYNOTE-D19.

Datopotamab deruxtecan was approved January 17, 2025 for pretreated HR-positive/HER2-negative disease based on TROPION-Breast01, with a first-line TNBC indication added May 22, 2026 based on TROPION-Breast02.

The two agents now overlap in first-line PD-1/PD-L1-ineligible TNBC. Sacituzumab govitecan is additionally available with pembrolizumab for PD-L1 CPS of 10 or higher and in later lines; datopotamab deruxtecan carries no combination indication in breast cancer.

As a single agent for the first-line treatment of adults with unresectable locally advanced or metastatic triple-negative breast cancer who are not candidates for PD-1 or PD-L1 inhibitor-based therapy.

In combination with pembrolizumab, or pembrolizumab and berahyaluronidase alfa-pmph, for the first-line treatment of adults with unresectable locally advanced or metastatic TNBC whose tumors express PD-L1 (CPS greater than or equal to 10) as determined by an FDA-authorized test.

For adults with unresectable locally advanced or metastatic TNBC who have received two or more prior systemic therapies, at least one for metastatic disease.

For adults with unresectable locally advanced or metastatic HR-positive, HER2-negative (IHC 0, IHC 1+, or IHC 2+/ISH-negative) breast cancer who have received endocrine-based therapy and at least two additional systemic therapies in the metastatic setting.

Trop-2-directed antibody-drug conjugate comprising a humanized anti-Trop-2 antibody linked by a hydrolyzable linker to SN-38, the active metabolite of irinotecan and a topoisomerase I inhibitor.

Carries a boxed warning for severe or life-threatening neutropenia and severe diarrhea.

Learn more about Sacituzumab Govitecan ›

For adults with unresectable or metastatic triple-negative breast cancer who are not candidates for PD-1/PD-L1 inhibitor therapy.

For adults with unresectable or metastatic HR-positive, HER2-negative (IHC 0, IHC 1+, or IHC 2+/ISH-negative) breast cancer who have received prior endocrine-based therapy and chemotherapy for unresectable or metastatic disease.

Trop-2-directed antibody-drug conjugate pairing a humanized anti-Trop-2 IgG1 antibody with the exatecan derivative DXd, a topoisomerase I inhibitor, through a tetrapeptide-based cleavable linker.

No boxed warning. Interstitial lung disease, ocular surface toxicity, and stomatitis are the notable labeled risks.

Learn more about Datopotamab Deruxtecan ›

1. Lai J, Deng S, Cao J, et al. Identification of biomarker associated with Trop2 in breast cancer: implication for targeted therapy. 

. 2024;15:413. doi:10.1007/s12672-024-01261-0

2. TRODELVY (sacituzumab govitecan-hziy) for injection. Prescribing information. Gilead Sciences, Inc; revised 06/2026. Accessed July 2026.

3. DATROWAY (datopotamab deruxtecan-dlnk) for injection. Prescribing information. Daiichi Sankyo, Inc; revised 05/2026. Accessed July 2026.

4. Garassino MC, Sands J, Paz-Ares L, et al. Normalized membrane ratio of TROP2 by quantitative continuous scoring is predictive of clinical outcomes in TROPION-Lung01. Presented at: IASLC 2024 World Conference on Lung Cancer; September 8, 2024; San Diego, CA.

5. US Food and Drug Administration. FDA approves sacituzumab govitecan-hziy as monotherapy and in combination with pembrolizumab for first-line treatment of triple-negative breast cancer. June 24, 2026. Accessed July 2026.

6. NCCN Clinical Practice Guidelines in Oncology (NCCN Guidelines®) for Breast Cancer V.5.2026. © National Comprehensive Cancer Network, Inc. 2026. All rights reserved. Accessed July 2026. To view the most recent and complete version of the guideline, go online to NCCN.org.

7. US Food and Drug Administration. FDA approves datopotamab deruxtecan-dlnk for unresectable or metastatic, HR-positive, HER2-negative breast cancer. January 17, 2025. Accessed July 2026.

8. US Food and Drug Administration. FDA approves datopotamab deruxtecan-dlnk for unresectable or metastatic triple-negative breast cancer. May 22, 2026. Accessed July 2026.

9. Trophoblast cell-surface antigen 2 (TROP2) expression in triple-negative breast cancer. BMC Cancer. 2022;22(1):1014. doi:10.1186/s12885-022-10076-7