In human fibroblast cultures derived from adults, clones of cells with a common chromosome rearrangement have been widely reported. Cells derived from one of these clones have been used in hybridization experiments using mouse cells to localize the genes for β-glucuronidase and mitochondrial malate dehydrogenase on human chromosome 7. The results of this study indicate that the genes for these isozymes are located on the region pter→q22 of human chromosome