Pheochromocytomas occurred in 8.3% of the p18−/− mice (2 of 24 mice aged 8 months or older), 23.8% of the p27−/− mice (5 of 21), 17% of the p18−/−p27+/− mice (4 of 24), 50% of the p18+/−p27−/− mice (three of six), and in 91.3% of the p18−/−p27−/− double knock-out mice (21 of 23) [7]. This evidence concerns the gene CDKN1B and hereditary pheochromocytoma-paraganglioma.