In conjunction with prior results that Rb1G/G; Cdkn1b-/- animals succumb to pituitary tumors [18], and that Rb1+/-; Cdkn1a-/- mice have accelerated pituitary tumorigenesis [30], these genetic crosses provide two simple conclusions relating to RB function in cell cycle and cancer susceptibility. This evidence concerns the gene CDKN1B and pituitary tumor.