Abstract The steady-state specific activity of hepatic phenylalanine hydroxylase in the 19–20-day-old dilute lethal mouse d 1 / d 1 is 2-fold higher than in its wild-type control D / D . The apparent K m values for phenylalanine and tetrahyfropteridine are identical for mutant and control enzymes. Acrylamide gel electrophoresis results in three peaks for each of the genotypes D / D , D / d 1 and d 1 / d 1 . The ratios of these peaks vary for each genotype. The slowest migrating peak is most abundant in d 1 / d 1 , however, it is also unstable.