CHEK2 and breast cancer: Monoallelic pathogenic and likely pathogenic variants (PVs) in CHEK2 (OMIM 604373) are associated with a predisposition to breast cancer and have been associated with colorectal, kidney, and thyroid cancers.1,2 Little is known about the phenotype associated with biallelic CHEK2 PVs; however, the phenotype is not known to be associated with severe childhood-onset conditions such as observed with biallelic ATM (OMIM 607585) or BRCA1/2 (BRCA1: OMIM 113705; BRCA2: OMIM 600185) PVs.