CHEK1 and neoplasm: The four genes upstream of the p53 signaling pathway were CDKN2A, CHEK1, CHEK2, and GORAB. Although CDKN2A regulates p53 expression via MDM2, it receives repression feedback from p53 and an inverse correlation between CDKN2A expression and p53 function has been reported in human tumor cell lines52.