The GALT enzyme activities of patients varied according to genotype with overlapping distributions: mean 3.9 μmol/h/g Hb for classic galactosemia patients (range, < 0.1–7.3), mean 6.1 μmol/h/g Hb for compound heterozygotes of Duarte and classic variants (range, 5.9–6.2), 12.5 μmol/h/g Hb for Duarte homozygotes, mean 14.5 μmol/h/g Hb for carriers of classic mutant alleles (range, 11.6–16.4), and mean 14.4 μmol/h/g Hb for carriers of Duarte variant alleles (range, 12.6–16.1). The gene discussed is GALT; the disease is classic galactosemia.