Multiple endocrine neoplasia type 1 (MEN1) and type 2 (MEN2) are major genetic disorders carrying a high risk of endocrine tumor development. The mutated genes were identified in 1993 (MEN2-RET) and 1997 (MEN1), enabling genetic testing and functional studies. Genetic analysis has led to new clinical and therapeutic strategies for MEN1/2 patients, and has improved our understanding of the pathways underlying the development of such tumors, which occur in an autosomal dominant manner and with high penetrance. The MEN1 gene encodes menin, a protein involved in many cell functions, such as transcription, genome stability, cell cycling and apoptosis. The MEN1 gene has 10 exons, and its exhaustive analysis in MEN1 patients helps guide their management. MEN2 is related to activating missense mutations in the RET protooncogene, which encodes a tyrosine kinase receptor (TKR). RET activation occurs upon autodimerization induced by the binding of specific ligands belonging to glial cell-derived neurotrophic factor-like family (GFL) proteins, regulated by coreceptors. The position of missense mutations--in the extracellular or intracellular TK domains--influences the aggressiveness of the most frequent malignancy, medullary thyroid carcinoma, establishing a genotype-phenotype correlation. We also briefly describe the genetic basis of three other inherited states predisposing individuals to endocrine tumors, namely Carney's syndrome, hyperparathyroidism type 2 (HRPT2) and familial isolated pituitary adenoma (FIPA), which are related to inactivating mutations in the PRKAR1-alpha, HRPT2 and AIP genes, respectively.