In this study, we detected homozygous mutations in the CYP17A1 gene (NM_000102.4:c.1053_1055delCCT; p.Leu353del; SCV001479329) in a 28-year-old female patient (46,XX) and her phenotypically female 30-year-old sister (46,XY) who had phenotypes consistent with combined 17-hydroxylase and 17,20-lyase deficiency. The gene discussed is CYP17A1; the disease is hyperinsulinemic hypoglycemia, familial, 4.