Between 5 and 10% of breast cancer cases are considered to result from hereditary predisposition. Germ-line mutations in BRCA1 and BRCA2 are responsible for an inherited predisposition of breast and ovarian cancer. Direct nucleotide sequencing is considered the gold standard technique for mutation detection for genes such as BRCA1 and BRCA2. In many laboratories that analyze BRCA1 and BRCA2, previous to direct sequencing, screening techniques to identify sequence variants in the PCR amplicons are performed. The mutations detected in these genes may be frameshift mutations (insertions or deletions), nonsense mutations, or missense mutations. The clinical interpretation of the mutation as the cause of the disease may be difficult to establish in the case of missense mutations. Only in 30–70% of the families in which a hereditary component is suspected, a mutation in BRCA1 and/or BRCA2 is detected. Negative results may be due to: wrong selection of the proband; mutations in the regulatory portion of the genes; gene silencing due to epigenetic phenomena; or large genomic rearrangements that produce deletions of whole exons. Another possibility that explains the lack of detection of alterations in BRCA1 or BRCA2 is the presence of mutations in undiscovered genes or in genes that interact with BRCA1 and/or BRCA2, which may be low-penetrance genes, like CHEK2.