Skeletal muscle phosphorylase kinase has the structure (alpha beta gamma delta)4 where the alpha and beta subunits are regulatory components, the gamma subunit possesses catalytic activity and the delta subunit is identical to the calcium binding protein calmodulin. A rabbit skeletal muscle cDNA for the gamma subunit has been used to map the human gene (PYKG1) to 7p12-q21, by analysis of somatic cell hybrids and in situ hybridisation. The data suggest that the skeletal muscle gamma subunit gene is located just above the centromere of chromosome 7, with further cross-hybridising sequences at 7q21 and 11p11-14. The liver gamma subunit is distinct and its mRNA does not cross-hybridize with the skeletal muscle gamma subunit cDNA. These results indicate that autosomal human phosphorylase kinase deficiencies affecting both liver and muscle are likely to be caused by a defect in the autosomally determined beta subunit, rather than the gamma subunit.