RB1 and hypoparathyroidism-retardation-dysmorphism syndrome: Tumors with germline (g) BRCA2 mutations or a homologous recombination deficiency (HRD) associated with gBRCA2, as well as baseline loss of heterozygosity (LOH) for RB1, exhibited a higher prevalence of RB1 loss-of-function mutations and demonstrated poor outcomes when treated with first-line CDK4/6i combinations.