In an infant with a D-bifunctional protein deficiency ( 261515 ), van Grunsven et al. (1998) identified a 46G-A transition in the HSD17B4 gene, resulting in a gly16-to-ser (G16S) substitution within an important loop of the Rossman fold forming the NAD(+)-binding site. Biochemical analysis showed that the 3-hydroxyacyl-CoA dehydrogenase activity of the D-bifunctional protein was completely inactive, whereas the enoyl-CoA hydratase component was active. Their findings showed that the D-bifunctional protein plays an essential role in the peroxisomal beta-oxidation pathway that cannot be compensated for by the L-specific bifunctional protein. Both parents were heterozygous for the mutation.