CDKN1B and multiple endocrine neoplasia: p27Kip1 can be considered a haploinsufficient tumor suppressor since monoallelic CDKN1B alteration, although rare, has been associated with a major risk of developing tumors and germinal monoallelic p27Kip1 mutation causes the multiple endocrine neoplasia, type 4 (MEN4) [293,294].