CDKN1B and pituitary tumor: Interestingly, the characteristic gigantism of p27 null mice was normalized by deletion of one Sox2 allele (Figure 4A; Figure S4A), as was also the case for the pituitary mass in young 3- to 6-month-old mice (at this age, p27 null mice present pituitary hyperplasia, but do not have pituitary tumors yet) (Figure 4B).