CDKN1A and breast cancer: Previous work by our and other groups has demonstrated that treatment of BC cells with 1,25(OH)2D3 inhibits BC cell proliferation by hampering the transition from G0/G1 to S phase through induction of cyclin-dependent kinase inhibitors (CDKIs) such as CDKN2D (p19), CDKN1A (p21), and CDKN1B (p27) (3).