Dissimilarly from the MEN1 gene, no somatic LOH at the 12p13 locus, containing the CDKN1B gene, was found in MEN4 PAs, suggesting this gene as a non-conventional tumor suppressor for which the down-regulation of p27Kip1 protein expression occurs at a post-transcriptional and/or post-translational level [22] by mechanisms that may regulate p27Kip1 stability, like as phosphorylation and ubiquitination [21]. This evidence concerns the gene MEN1 and neoplasm.