A human-mouse hybrid, DUR 4 (Solomon et al., 1976), containing a human X/15 translocation chromosome and also chromosome 5, among other human chromosomes, was used in a double selection system to obtain hybrids of four different types: X/15+ 5+, X/15– 5+, X/15+ 5, and X/15– 5–. Standard positive and negative selection systems were used for the X chromosome, and negative selection for chromosome 5 was done with diphtheria toxin. The assignment of HEXB and the diphtheria toxin sensitivity gene to chromosome 5 was confirmed. HEXA was present only when both the X/15 chromosome and chromosome 5 were present. A “HEXA-like” band segregated with chromosome 15 (or X/15) but independently of chromosome 5. This component, unlike HEXA, does not contain human HEXβ