CDKN1B and neoplasm: Although tumor suppressor functions of CDKN1B are well known from mouse studies, it is notable that I-NETs have not been reported in CDKN1B mutant mice, nor have I-NETs been reported in patients with MEN4 syndrome, in which patients develop neuroendocrine tumors in multiple endocrine organs due to rare allelic forms of CDKN1B [8].