RET and medullary thyroid gland carcinoma: It is caused by germline RET mutations, which can be inherited or occur de novo.<h4>Objective</h4>This study aimed to define the prevalence and genetics of de novo MEN2 syndromes, which are not yet fully understood, and to characterize the parental origin of the RET de novo mutation.<h4>Methods</h4>We selected 152 of 215 families with hereditary MTC.