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PEG1/MEST gene has been known to be an imprinting gene, which is associated with growth of mesodermal origin cells. Its expression was also reported to be increased in leiomyoma. Several reports showed that loss of imprinting is associated with carcinogenesis in some types of cancer. The purpose of this study was to investigate whether overexpression of PEG1/MEST gene in leiomyoma is associated with loss of imprinting of the gene (biallelic), or whether the overexpression occurs while maintaining the imprinting (monoallelic). We investigated the expression and the imprinting status of PEG1/MEST and its isoforms in samples from 25 patients with uterine leiomyomas as well as in matched normal myometrial tissue. The isoform 1 transcripts were found to be more increased in uterine leiomyomas, compared to myometrium. However, there was no difference in the mRNA levels of isoform 2 between normal myometrium and leiomyoma. All normal myometrial tissues and 19 of 20 leiomyomas showed monoallelic expression of PEG1/MEST. Thus, these data demonstrated that tumorigenesis of leiomyoma is associated with overexpression of isoform 1 of PEG1/MEST gene, but not with loss of imprinting of the gene. Copyright © 2010 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]