Tissue was collected from 89 patients with HR+/HER2- MBC, including those with recurrent and/or metastatic disease, receiving palbociclib plus an aromatase inhibitor or fulvestrant...Likewise, the presence of mutated BRCA1/2 was associated with shorter PFS (HR = 2.67; p = 0.012, q = 0.049). Mutation of TP53 (HR = 3.92; p < 0.001, q < 0.001) and APOBEC signature S13 (HR = 3.19; p = 0.002, q = 0.012) were associated with shorter PFS.