The 46,XX testicular disorder of sex development (DSD), also known as 46,XX male 
syndrome, is a rare form of DSD and clinical phenotype shows complete sex 
reversal from female to male. The sex-determining region Y (SRY) gene can be 
identified in most 46,XX testicular DSD patients; however, approximately 20% of 
patients with 46,XX testicular DSD are SRY-negative. The SRY-box 9 (SOX9) gene 
has several important functions during testis development and differentiation in 
males, and overexpression of SOX9 leads to the male development of 46,XX gonads 
in the absence of SRY. In addition, SOX9 duplication has been found to be a rare 
cause of 46,XX testicular DSD in humans. Here, we report a 4.2-year-old 
SRY-negative 46,XX boy with complete sex reversal caused by SOX9 duplication for 
the first time in Korea. He showed normal external and internal male genitalia 
except for small testes. Fluorescence in situ hybridization and polymerase chain 
reaction (PCR) analyses failed to detect the presence of SRY, and SOX9 
intragenic mutation was not identified by direct sequencing analysis. Therefore, 
we performed real-time PCR analyses with specific primer pairs, and duplication 
of the SOX9 gene was revealed. Although SRY-negative 46,XX testicular DSD is a 
rare condition, an effort to make an accurate diagnosis is important for the 
provision of proper genetic counseling and for guiding patients in their 
long-term management.