Abstract: Objective:A study was conducted to investigate the biological characteristics of familial medullary thyroid carcinoma (FMTC) caused by rare RET proto-oncogene mutation Y 606 C and to discuss treatment options. Methods:Systemic medical data, in -cluding RET germline mutation screening, biochemical testing, imaging examinations, clinical management, and postoperative follow-up, were obtained from 10members of the3-generate FMTC family. These data were confirmed previously. The clinical features of the above-mentioned cases were summed up. Results:RET screening showed heterozygous missense mutation of Y 606 C on exon10in 6 cases. Among the 6 carriers, 3 cases (including the index patient) have been affected by MTC. The mean age of these patients at the time of diagnosis was 50. 0 years old (in the range of 47- 53years old). The manifestations associated with MTC included thyroid nod -ules and elevated levels of calcitonin. Two patients accepted total thyroidectomy and one patient underwent lobectomy. MTC was con-firmed by postoperative pathology and all tumors were early stage without regional lymph node metastasis. Biochemical cure was achieved during the follow-up. The mean age of three mutation carriers was33. 0 years (in the range of 15- 55years). Among the three carriers, multiple thyroid lesions have been detected by ultrasound in a55-year-old female and a 15-year-old male. One carrier reject -ed further clinical examinations. The other two carriers' serum calcitonin levels were normal. Thus, close observation was recommended. Conclusion: RET mutation of Y606 C might lead to pathogenesis of FMTC, which is associated with older age of onset, earlier tumor stage, higher biochemical cure rate, and better prognosis. Molecular screening of the RET gene in FMTC family members, combined with ultrasound and thyroid calcitonin examinations, may be helpful in the early diagnosis of MTC. For asymptomatic Y 606 C mutation carriers, management should be individualized based on calcitonin level and neck ultrasound.