In order to understand the histogenesis of precancerous and cancerous lesions, we investigated the clonality of normal single fundic gland and single pyloric gland using X-chromosome inactivation. Sixty percent of single fundic glands were cell populations methylated at the same alleles of the X-chromosome (homotypic) and 40% consisted of mixed cells methylated at different alleles of the X-chromosome (heterotypic). Among 28 fundic glands in which the clonality of the upper parts and that of the lower parts were analyzed separately, 11 glands were heterotypic in either part and 4 glands were homotypic with different allelic methylation in the upper part and in the lower part. Ninety-six percent of the single pyloric glands were homotypic and 4% were heterotypic. The results show the differences in the development of the fundic glands and the pyloric glands.