D-bifunctional protein deficiency (D-BPD) is a rare fatal autosomal recessive peroxisomal disorder caused by biallelic pathogenic mutations in the hydroxysteroid 17-beta dehydrogenase 4 (HSD17B4) gene; it is characterized by hypotonia, seizures, and facial dysmorphisms during the neonatal period. Here, HSD17B4 is linked to d-bifunctional protein deficiency.