Lee et al. (1992) sought to clone a human galactokinase gene by its ability to substitute for the enzyme in yeast. They used a method for identifying mammalian cDNAs by complementation or other functional activity in yeast. They unexpectedly found that the gene which complemented a galactokinase-deficient strain of Saccharomyces cerevisiae mapped not to chromosome 17 but to chromosome 15, thus calling into question which of the genes, GK1 on 17 or GK2 on 15, is the site of the mutation in the galactokinase deficiency form of galactosemia. The strategy they used might be adopted for cloning various human disease genes affecting intermediary metabolism for which yeast mutants are known. (The mapping to chromosome 15 was done by analysis of genomic DNA from a panel of human-rodent somatic cell hybrids using PCR.)