An apparently balanced de novo translocation between chromosomes X and 20, 46,X,t(X;20)(Xp20q;Xq20p), was identified in a severely retarded 13-year-old female with macrocephaly, bilateral overfolded pinnae, elbow contractures, clinodactyly, and seizures. BudR-pulse studies show the normal X chromosome to be late replicating in both lymphocytes (50 cells) and skin fibroblasts (25 cells). An HPRT deficient Chinese hamster line was fused with lymphocytes from the patient, and hybrid lines were derived in HAT medium. Cytogenetic and biochemical analyses of these hybrid lines show that the locus for adenosine deaminase is in the cen leads to qter region and that the locus for inosine triphosphatase is in the pter leads to cen region of human chromosome 20.