These results indicate that variant patients have a higher residual capacity to metabolize galactose compared to classical patients and that the GI in fibroblasts (defined as the ratio of [U13C]-Gal-1-P/[13C6]-UDP-galactose) can be used as a measure for the severity of the GALT deficiency and residual galactose metabolism in galactosemia patients. The gene discussed is GALT; the disease is galactosemia.