The PTEN (MMAC1/TEP1) tumor suppressor gene was recently isolated and mapped to 
human chromosome band 10q23. Homozygous deletions and mutations of PTEN were 
observed in cell lines and sporadic cancers of the breast, kidney, and central 
nervous system. Germline mutations in PTEN were recently found in Cowden 
disease, an autosomal dominant inherited syndrome, previously mapped to 
chromosome bands 10q22-23. This disease is associated with a wide variety of 
malignancies and hamartomas of ectodermal, mesodermal, and endodermal origin. 
The most common neoplasms in Cowden disease patients arise in the breast, skin, 
and thyroid (follicular subtype). To determine the involvement of PTEN in 
sporadic follicular thyroid tumors, we first analyzed sporadic follicular 
adenomas and carcinomas for deletions of the PTEN gene. Loss of heterozygosity 
was found in 7/26 (27%) follicular carcinomas and 2/27 (7%) follicular adenomas, 
one of which was a small hemizygous deletion (approximately 3 cm). Sequence 
analysis of the entire PTEN coding region revealed two mutations in carcinomas 
with 10q loss. Our findings suggest that the PTEN tumor suppressor gene is 
occasionally inactivated in sporadic follicular thyroid tumors.