The human spasmolytic protein, SML1 hSP, an inhibitor of spasmolytic activity and gastric acid secretion in the pig, has been shown to exhibit homology to the pS2 protein, an estrogen-dependent breast cancer marker. Moreover, SML1 hSP and pS2 are expressed at the same localization in the normal stomach and during healing of the gastrointestinal tract. Here we report the chromosomal localization, obtained by in situ hybridization, of the hSP gene (SML1) to chromosome 21 at 21q22.3. Using pulsed-field gel electrophoresis, we found SML1 to be within 230 kb of the BCEI pS2 gene. © 1992.