We report a unique and complex karyotypic rearrangement involving chromosomes X, 3, 7, and 21. Blood cells and fibroblasts from the proband do not express the maternal allele for glucose-6-phosphate dehydrogenase (G6PD), providing biochemical evidence for nonrandom expression of X-linked genes in balanced X-autosome translocations. The break point on the X chromosome, at the junction of Xq27-Xq28, separates the loci for hypoxanthine phosphoribosyltransferase (HPRT) and G6PD. Studies of mouse-human hybrids derived from the proband's cells indicate that G6PD, at q28, is clearly distal to all other X loci now assigned. From these and previous studies, we can localize HPRT to that segment between Xq26 and Xq27. The studies also provide further evidence for the stability of the inactive X phenotype in hybrid cells.