[Show abstract][Hide abstract] ABSTRACT: Objective: Medullary Thyroid Carcinoma (MTC) is hereditary in 25% of cases, and occurs primarily in an autosomal dominant manner. Thus, siblings and offspring of a patient diagnosed with a germline mutation are at a 50% risk for carrying the same mutation. Germline mutations of RET proto-oncogene is the known cause of almost all hereditary MTCs. Mutation analysis of RET is necessary for all apparently sporadic MTC patients. Cases Presentation: In this study, three siblings diagnosed with MTC in a family were analyzed for RET mutations. The result of direct DNA sequencing showed Cys634Arg point mutation in all 3 of them. Screening of their offspring and 4th sibling resulted in identification of a 3-year-old boy as a mutation carrier. According to general consensus, he is at second risk level, which qualifies him for prophylactic total thyroidectomy. Conclusion: Molecular analysis of RET proto-oncogene should be done in a routine manner to differentiate hereditary MTC patients from sporadic ones. In case of finding a germline mutation, all siblings and offspring of the mutation carrier should be screened and asymptomatic carriers referred to total prophylactic thyroidectomy.