Coreactivation of four inactive X genes in a hamster x human hybrid and persistence of late replication of reactivated X chromosome.

Hamster-human hybrids which contained an inactive human X chromosome were treated by 5-azacytidine. Hypoxanthine guanine phosphoribosyltransferase derepressed hybrids were selected and derepression of three other loci, phosphoglycerate kinase, alpha-galactosidase, and glucose-6-phosphate dehydrogenase were studied. Among 32 hybrids selected for hypoxanthine guanine phosphoribosyltransferase, two were found to be reactivated at four X loci. The independence or nonindependence of the reactivation events will be discussed. No correlation was found between the time of replication and the expression or nonexpression of the X chromosome genes: X chromosomes reactivated at four loci remained late replicating; conversely early replication can exist without the expression of some X genes.