Based on whole-exome sequencing (WES) results, we employed validated approaches to determine whether the genomic hallmark features of HRD15 are present in PALB2-associated breast cancers irrespective of the presence of a second hit affecting PALB2. Moreover, as an exploratory, hypothesis-generating analysis, we compared the genomic landscape of breast cancers from pathogenic PALB2 germline mutation carriers to that of breast cancers arising in BRCA1 or BRCA2 germline mutation carriers, and non-BRCA1/2/PALB2-associated breast cancers. The gene discussed is BRCA1; the disease is breast cancer.