RET germline mutations were found to predispose to the development of three variants of multiple endocrine neoplasia type 2, MEN2A, MEN2B, and familial medullary thyroid carcinoma (FMTC). We have screened for RET mutations at exons 10, 11, 13, and 16 in leukocyte DNA extracted from 37 individuals, and have identified RET germline mutations in 12 affected individuals from 9 unrelated families. No RET germline mutation was found in 19 individuals with apparent sporadic diseases. We have also screened for RET mutations at exons 10, 11, and 16 in tumor DNA extracted from 13 freshly frozen medullary thyroid carcinomas (MTC). RET mutation was detected in every tumor, either inherited or sporadic, indicating that RET plays an important role in the development of both inherited and sporadic MTC. We initially screened for RET mutations by direct DNA sequencing of the genomic PCR products amplified from patients' leukocyte or tumor DNA. Recently, we utilized the "Cold SSCP" method, nonradioactive single-stranded conformation polymorphism analysis, to screen for RET mutations and have identified a novel mutation, a 6-bp deletion preceding the cysteine-634, in a sporadic MTC.