RB1 and breast cancer: This possibility is also supported by frequent lesions in the RB/E2F pathway in breast cancer, including loss of RB1, CDKN1B (encoding p27), and CDKN2A as well as amplification of CCND1 (Angus et al., 2019; Bertucci et al., 2019; Cancer Genome Atlas Network, 2015; Ertel et al., 2010; Nik-Zainal et al., 2016; Cancer Genome Atlas Network, 2012).