Inactivating mutations of the 5α-steroid reductase type-2 (SRD5A2) gene result 
in a broad spectrum of masculinization defects, ranging from a male phenotype 
with hypospadias to a female phenotype with Wolffian structures. Molecular 
studies of the SRD5A2 revealed a new heterozygous gene variant within the coding 
region that results in phenotypic expression. A c.92C>T transition changing 
serine to phenylalanine at codon 31 of exon 1 (p.Ser31Phe) was identified in a 
patient with 46,XY disorder of sexual development who displayed glandular 
hypospadias with micropenis and bilateral cryptorchidism. The restoration of the 
p.Ser31Phe mutation by site-directed mutagenesis and transient expression assays 
using cultured HEK-293 cells showed that this novel substitution does not 
abolish but does deregulate the catalytic efficiency of the enzyme. Thus, the 
maximum velocity (V max) value was higher for the mutant enzyme (22.5 ± 6.9 nmol 
DHT mg protein(-1) h(-1)) than for the wild-type enzyme (9.8 ± 2.0 nmol DHT mg 
protein(-1) h(-1)). Increased in vitro activity of the p.Ser31Phe mutant 
suggested an activating effect. This case provides evidence that heterozygous 
missense mutations in SRD5A2 may induce the abnormal development of male 
external genitalia.