Galactosemia is caused by defects in the galactose metabolic pathway, which consists of three enzymes, including UDP-galactose-4-epimerase (GALE). We previously reported nine mutations in Korean patients with epimerase-deficiency galactosemia. In order to determine the functional consequences of these mutations, we expressed wild-type and mutant GALE proteins in 293T cells. GALE(E165K) and GALE(W336X) proteins were unstable, had reduced half-life, formed aggregates and were partly degraded by the proteasome complex. When expressed in GALE-null ldlD cells GALE(E165K), GALE(R239W), GALE(G302D) and GALE(W336X) had no detectable enzyme activity, although substantial amounts of protein were detected in western blots. The relative activities of other mutants were lower than that of wild-type. In addition, unlike wild-type, GALE(R239W) and GALE(G302D) were not able to rescue galactose-sensitive cell proliferation when stably expressed in ldlD cells. The four inactive mutant proteins did not show defects in dimerization or affect the activity of other mutant alleles identified in patients. Our observations show that altered protein stability is due to misfolding and that loss or reduction of enzyme activity is responsible for the molecular defects underlying GALE-deficiency galactosemia.