**4. 17β-hydroxysteroid dehydrogenase type 3 deficiency**

17β-hydroxysteroid dehydrogenase type 3 (17β-HSD3) deficiency (OMIM #264300), originally described as 17-ketosteroid reductase deficiency (Saez et al., 1971), is an autosomal recessive disorder which represents the most common defect of the biosynthesis of T in 46,XY DSD (Bertelloni et al., 2004; Mendonca et al., 2000). This disorder is due to an impaired conversion of Δ4-A into T in the testes (Bertelloni et al., 2009; Faienza et al., 2008). Deficiency in the 17-HSD3 enzyme can be caused by either homozygous or compound heterozygous mutations in the *HSD17B3* gene (Geissler et al., 1994). Mutations in the *HSD17B3* gene confer a spectrum of 46,XY disorders of sexual organ development ranging from completely undervirilized external female genitalia (Sinnecker type 5), predominantly female (Sinnecker type 4), ambiguous (Sinnecker type 3), to predominantly male with micropenis and hypospadias (Sinnecker type 2) (Boehmer et al., 1999; Sinnecker et al., 1996). The most frequent presentation of 17β-HSD3 deficiency is a 46,XY individual with female external genitalia, labial fusion and a blind ending vagina, with or without clitoromegaly (Sinnecker types 5 and 4).
