Drug developers are mounting a poly(ADP-ribose) polymerase (PARP) inhibitor comeback, and a few are now experimenting with a more comprehensive companion diagnostic to smooth the way. BioMarin, of San Rafael, California, announced in November that it will use Myriad Genetics' novel homologous recombination deficiency (HRD) diagnostic to identify likely responders to its experimental PARP inhibitor BMN 673, currently in phase 3. Clovis Oncology, of Boulder, Colorado, is working on a similar strategy with diagnostic firm Foundation Medicine, of Cambridge, Massachusetts, to develop a PARP inhibitor. “For the pharmaceutical industry, these HRD tests could provide an opportunity to expand the market and license of their drugs,” says Jonathan Ledermann, an oncologist at University College London, who has collaborated on PARP inhibitor trials. PARP inhibitors block DNA repair enzymes used by cancer cells to repair themselves. For years PARP inhibitor developers have used BRCA1 and BRCA2 assays to identify tumors with dysfunctional DNA repair pathways that are likely to respond to the therapy. BioMarin and four other companies—AstraZeneca of London; Tesaro of Waltham, Massachusetts; AbbVie of North Chicago, Illinois; and Teva of Petach Tikva, Israel—have incorporated Salt Lake City, Utah–based Myriad's older BRACAnalysis test into the clinical development programs of their experimental PARP inhibitors. Myriad's HRD test is a gene-agnostic assay that looks at 50,000 SNPs to look for evidence of genomic disrepair. Early data suggest that the broad HRD approach could capture twice as many responders as a BRCA-specific analysis, adds Ledermann.