{"entity":{"id":"rad51-foci-assay","kind":"technology","name":"RAD51 foci assay (functional HRD test)","aka":["RAD51 assay","RAD51 immunofluorescence","functional homologous recombination test"],"tldr":"A microscope test on an ordinary tumour biopsy that counts RAD51 repair spots in dividing cells; few spots means the tumour cannot repair DNA by homologous recombination right now, which is what PARP inhibitors exploit.","summary":"What it measures. RAD51 is the protein that carries out homologous recombination repair. When repair is working, cells that have replicated their DNA show nuclear RAD51 foci after damage; when BRCA1, BRCA2, PALB2 or another repair gene is lost, the foci do not form. The assay stains a routine formalin-fixed slide for RAD51 and geminin (a marker of the cells that should be repairing) and reports the share of geminin-positive cells with RAD51 foci. A low score is a functional read-out of homologous recombination deficiency at the moment of the biopsy.\n\nWho should have it. It is being studied in the same settings as genomic HRD tests: high-grade serous ovarian cancer, triple-negative and BRCA-associated breast cancer, and metastatic prostate cancer, where the question is whether a PARP inhibitor or platinum will work. Unlike a genomic scar, which stays behind after resistance has developed, RAD51 foci return when repair is restored, so the assay may also identify patients whose tumours have become resistant.\n\nEvidence. The Vall d'Hebron group showed that a RAD51 score measured in routine samples predicted PARP inhibitor response in patient-derived xenografts beyond BRCA mutation (EMBO Molecular Medicine 2018) and later linked low RAD51 to benefit from platinum in the GeparSixto breast cancer trial. Prospective trials selecting patients on RAD51 are ongoing. The assay is a research and academic laboratory test; no version has regulatory clearance as a companion diagnostic.\n\nWhat changes. Today, nothing formally: treatment decisions still rest on BRCA sequencing and genomic scar scores. If validated, a low RAD51 score would widen PARP inhibitor use to tumours with no BRCA mutation and a negative scar score, and a high score after progression would argue against continuing them. Cost is that of an immunohistochemistry stain plus specialised scoring, and availability is limited to research centres.","status":"emerging","asOf":"2026-09-17","links":[{"label":"EMBO Molecular Medicine 2018: A RAD51 assay feasible in routine tumor samples calls PARP inhibitor response beyond BRCA mutation","url":"https://doi.org/10.15252/emmm.201809172"}],"tags":[],"related":["companion-diagnostic","intodna","b-biomarker-validation"],"cancers":["high-grade-serous-ovarian-cancer","ovarian","tnbc","breast-cancer","prostate"],"sections":["diagnostics"],"technologies":["hrd-testing","hrd-genomic-scar-scores","stride-dna-break-detection","parp-inhibitor","histopathology-ihc"],"targets":["brca","parp"],"drugs":["olaparib","niraparib","rucaparib","talazoparib","carboplatin","cisplatin"],"companies":[],"institutions":[],"pathways":[],"terms":["hrd"],"trials":[],"people":[],"bottlenecks":[],"keyPapers":[],"journals":[],"dependsOn":[],"notes":[],"principle":"Immunofluorescence for RAD51 and geminin on formalin-fixed tumour sections; the fraction of geminin-positive (S/G2) cells with five or more nuclear RAD51 foci scores current homologous recombination capacity.","strengths":["Reads repair capacity at the time of biopsy rather than a permanent scar","Works on routine formalin-fixed samples without sequencing","May detect restored repair when resistance develops"],"limitations":["Research use only, no regulatory clearance","Needs enough dividing tumour cells and careful scoring","Prospective proof that it selects responders is still being collected"],"since":2018},"route":"/technologies/rad51-foci-assay/","neighbours":{"technology":[{"id":"companion-diagnostic","kind":"technology","name":"Companion diagnostics","route":"/technologies/companion-diagnostic/"},{"id":"histopathology-ihc","kind":"technology","name":"Histopathology & immunohistochemistry","route":"/technologies/histopathology-ihc/"},{"id":"hrd-testing","kind":"technology","name":"HRD & BRCA testing","route":"/technologies/hrd-testing/"},{"id":"hrd-genomic-scar-scores","kind":"technology","name":"HRD genomic scar scores (GIS, LOH, HRDetect)","route":"/technologies/hrd-genomic-scar-scores/"},{"id":"parp-inhibitor","kind":"technology","name":"PARP inhibitors","route":"/technologies/parp-inhibitor/"},{"id":"stride-dna-break-detection","kind":"technology","name":"STRIDE DNA break detection (intoDNA)","route":"/technologies/stride-dna-break-detection/"}],"company":[{"id":"intodna","kind":"company","name":"intoDNA","route":"/companies/intodna/"}],"bottleneck":[{"id":"b-biomarker-validation","kind":"bottleneck","name":"Biomarkers are not validated or standardised","route":"/bottlenecks/b-biomarker-validation/"}],"cancer":[{"id":"breast-cancer","kind":"cancer","name":"Breast cancer (all types)","route":"/cancers/breast-cancer/"},{"id":"high-grade-serous-ovarian-cancer","kind":"cancer","name":"High-grade serous ovarian cancer","route":"/cancers/high-grade-serous-ovarian-cancer/"},{"id":"ovarian","kind":"cancer","name":"Ovarian cancer","route":"/cancers/ovarian/"},{"id":"prostate","kind":"cancer","name":"Prostate cancer","route":"/cancers/prostate/"},{"id":"tnbc","kind":"cancer","name":"Triple-negative breast cancer (TNBC)","route":"/cancers/tnbc/"}],"section":[{"id":"diagnostics","kind":"section","name":"Diagnostics & Biomarkers","route":"/fronts/diagnostics/"}],"target":[{"id":"brca","kind":"target","name":"BRCA1 / BRCA2 (HRD)","route":"/targets/brca/"},{"id":"parp","kind":"target","name":"PARP","route":"/targets/parp/"}],"drug":[{"id":"carboplatin","kind":"drug","name":"Carboplatin","route":"/drugs/carboplatin/"},{"id":"cisplatin","kind":"drug","name":"Cisplatin","route":"/drugs/cisplatin/"},{"id":"niraparib","kind":"drug","name":"Niraparib","route":"/drugs/niraparib/"},{"id":"olaparib","kind":"drug","name":"Olaparib","route":"/drugs/olaparib/"},{"id":"rucaparib","kind":"drug","name":"Rucaparib","route":"/drugs/rucaparib/"},{"id":"talazoparib","kind":"drug","name":"Talazoparib","route":"/drugs/talazoparib/"}],"term":[{"id":"hrd","kind":"term","name":"Homologous recombination deficiency (HRD)","route":"/terms/hrd/"}]}}