Homologous recombination (HR) mutations can be classified into three distinct functional groups: those with germline plus somatic mutations; somatic-only, biallelic mutations; and somatic-only, monoallelic mutations. Subgroups could be further subdivided according to the type of mutation involved (frameshift, nonsense, missense, and copy loss), leading to different HR deficiency (HRD) status and clinical benefit of PARP inhibition. A less robust response can be reasonably expected in the somatic-only, monoallelic group, as well as in the somatic-only, biallelic group if somatic alterations are heterogeneous. PARP, poly(ADP-ribose) polymerase.