In 2025, the diagnosis of Δ4‐3‐oxosteroid 5β‐reductase deficiency requires both the demonstration that the major bile acids in the urine are conjugates of 7α‐hydroxy‐3‐oxo‐4‐cholenoic acid and 7α,12α‐dihydroxy‐3‐oxo‐4cholenoic acid and the demonstration of biallelic pathogenic mutations in AKR1D1. The gene discussed is AKR1D1; the disease is hyperinsulinemic hypoglycemia, familial, 4.