To investigate the functional change of SRD5A2 gene mutations identified in patients with 5α-reductase type 2 deficiency. Three unrelated subjects born with ambiguous genitalia were included. All patients were initially reared as girls, but they gradually exhibited variable degrees of virilization at puberty without breast development, followed by a change of gender role. Sequencing analysis of the SRD5A2 gene was performed and enzymatic activities of 5α-reductase type 2 were assessed. Three compound heterozygous mutations in the SRD5A2 gene were identified: patient 1 with p. G203S/c. 655delT, patient 2 with p. Q6X/p. G203S, and patient 3 with p. G203S/c. 755_756insT. Heterozygosity for the p. V89L polymorphism was also found in patients 2 and 3. The c. 655delT mutation led to a complete loss of the enzymatic activity, whereas mutants p. G203S and c. 755_756insT resulted in a reduction of the enzymatic activities by 60 and 90%, respectively. Two frameshift heterozygous mutations, c. 655delT and c. 755_756insT, led to a dramatic reduction in 5α-reductase activity, and the latter had not been reported previously. In addition, the heterozygous mutation (p. G203) identified in the 3 patients presumably suggests a founder effect in the Chinese population and may explain the similarity in phenotype among the patients.