Two 3-oxo-Δ4 fetal bile acids, 3-oxachola-4,6-dien-24-oic acid (1) and 7α-hydroxy-3-oxochol-4-en-24-oic acid (2), occur normally in the human fetus but remain elevated in neonates and children with severe cholestatic liver disease due to an autosomal recessive inborn error of metabolism affecting Δ4-3-oxo-steroid 5β-reductase (AKR1D1). The gene discussed is AKR1D1; the disease is Cholestatic liver disease.