Only homozygous, not heterozygous, Brca1 knockout mice develop breast cancers (Evers and Jonkers, 2006), whereas breast (and other) cancers develop in Becn1 heterozygous knockout mice (Qu et al., 2003, Yue et al., 2003, Cicchini et al., 2014). This evidence concerns the gene BRCA1 and breast cancer.