p21Cip1 is overexpressed in hormone-secreting PAs, especially in GH-PAs,[34] and no mutations have been observed in the encoding gene.[35] Chesnokova et al[36] reported that p21Cip1 suppressed pituitary tumor growth, which was correlated with hormonal secretion type, and hypothesized that p21Cip1 overexpression accounted for the very low incidence of PCs. This evidence concerns the gene CDKN1A and pituitary tumor.