Fig. 1  Pituitary tumorigenesis in Men1+/− mice is abrogated in the Cdk4−/− background, but not in the Cdk2-/- background. (A) Macroscopic pictures of pituitary organs or tumors from mice with the indicated genotypes. Blue arrows indicate non-tumorigenic pituitaries in Men1+/+ (wild-type) mice in the Cdk wild-type (i.e., Cdk4+/+; Cdk2+/+), Cdk4−/− or Cdk2−/− background and in Men1+/− mice in the Cdk4−/− backgrounds. Note that pituitaries of mice in the Cdk4−/− backgrounds are smaller with hypoplastic changes as we reported previously. Large pituitary tumors in Men1+/−; Cdk wild-type and Men1+/−;Cdk2−/− mice are shown as large masses in dark red color occupying much of the fields. (B) Incidences of pituitary tumors determined at 15 months of age. The total numbers of Men1+/+mice, Men1+/− mice, Men1+/−;Cdk4−/− mice and Men1+/−;Cdk2−/− mice examined for tumorigenesis were 12, 31, 30 and 29, respectively (see Results for details). *, p=0.001; **, p=0.004; ***, p<0.0001; N.S., not significant.