CDKN1A and neoplasm: 1,25-Dihydroxyvitamin D3 [1,25(OH)2D3, calcitriol] binds to a corresponding intranuclear receptor and interacts with various cell cycle regulators identified in numerous genes involved in cellular growth, differentiation, apoptosis, and invasion by tumor cells such as the human p21/WAF1, cyclin A and cyclin E, human c-fos and c-myc, and the human retinoblastoma gene [8].