RB1 and breast cancer: Aberrant CDK4 amplification in malignant tissues has been reported to be involved in the development and progression of various cancers including liposarcoma (Creytens et al., 2015), glioblastomas (Schmidt et al., 1994), breast cancer (Piezzo et al., 2020), ovarian cancer (Masciullo et al., 1997), and melanoma (Muthusamy et al., 2006) through the cyclin D1–CDK4/6–Rb pathway.