Analysis of CHO electrophoretic mobility shift mutants for six enzyme loci ( LDHA , GAA, IDH2 , ME1, PGM3, and MPI) that have been previously mapped to Chinese hamsters chromosomes 3 and 4 indicated that each of these loci, with the exception of IDH2 , are functionally dizygous in CHO. Segregation analysis of CHO X mouse somatic cell hybrids allowed regional gene mapping assignments for a total of eight Chinese hamster chromosome 3- or 4-derived marker loci (the above six, plus APRT and PKM2) to CHO chromosomes Z3 , Z4 , Z5 , and Z7 . For seven of these enzyme loci (all but IDH2 ), two alleles are expressed in CHO cells, each segregating with a different Z-group chromosome. These gene mapping assignments confirm genetically that CHO chromosomes Z3 , Z4 , Z5 , and Z7 are, in fact, derived from Chinese hamster chromosomes 3 and 4, and provide insight into the effects of chromosomal rearrangements on gene expression and hemizygosity in CHO cells.