There is a lack of effective treatments for immunotherapy-refectory melanoma. Although PARP inhibitors (PARPi) are an effective treatment strategy in cancers with homologous recombination deficiency (HRD), determining HRD status is challenging in melanoma. Here, we chart the longitudinal relationship between PARPi response and HRD scores derived from genome-wide loss of heterozygosity (LOH) in 4 patients with metastatic melanoma. When next examining 933 melanoma cases, using an updated threshold, we observed HRD-related LOH (HRD-LOH) in nearly one-third of all cases compared with <10% using traditional gene panels. Taken together, HRD-LOH in refractory melanoma is both a common occurrence and a potential biomarker for response to PARPi.

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