CDKN1A and neoplasm: Although p21 is a direct p53 target (28,29), it is known to be regulated by many other tumour suppressors, such as BRCA1 (30), CHK2 (31), FOXP3 (32), transforming growth factor-β (33) and importantly repressed by oncogenes, such as c-myc (34) and Gfi1 (35).