The clonality of gastric glands remains a controversial topic. Chimeric mouse studies suggested that all gastric glands were monoclonal. However, using the X-linked transgenic mouse model, we have suggested that most glands are polyclonal during development and that the fraction of monoclonal glands increases after birth. Nevertheless, a fraction of glands is perpetually polyclonal even in the adult murine stomach. To examine the existence of gastric polyclonal glands in the adult mouse, we studied an X-linked intrinsic polymorphic gene, pgk-1, in 6 week-old female mice heterozygous for the X-linked pgk-1a and pgk-1b. The sequence containing the seventh HpaII site in the promoter region of the gene and the polymorphic sites was utilized. Twenty-four of 225 fundic glands (10.7%) and 3 of 167 pyloric glands (1.8%) were clonally heterotypic. Only 0.6% of colonic crypts were heterotypic. Clonally heterotypic glands with inactive X-specific methylation were present in the adult murine stomach.