Using a rat histidine decarboxylase (HDC) cDNA probe, we have mapped the HDC gene by in situ hybridization to the q15-q21 region of human chromosome 15 and to the E5-G region of murine chromosome 2. These localizations strengthen a syntenic group conserved between human chromosome 15 and mouse chromosome 2. The localization of the HDC gene on the human chromosome 15 map shows that it is not included within the Prader-Willi Syndrome region (PWCR).