CDKN1A (p21) is transcriptionally activated by TP53 (p53) after DNA damage (el-Deiry et al. 1993). EP400 (p400) binds to a CDKN1A promoter region that overlaps with the distal TP53-binding site and can co-localize with TP53 on CDKN1A promoter. The presence of EP400 results in the downregulation of CDKN1A transcription without affecting the phosphorylation of TP53 on serine S15 (Chan et al. 2005).