Steroidogenic acute regulatory protein (STAR), is a rapidly synthesized labile mitochondrial phosphoprotein whose expression, activation and extinction is regulated by protein kinase A (PKA) and protein kinase C (PKC), as well as a host of other signaling pathways. STAR is primarily present in steroid-producing cells, including Leydig cells in the testis, theca cells and luteal cells in the ovary and adrenal cells in the adrenal cortex. Low level of STAR expression in other tissues that produce steroid hormones for local use have been reported. STAR is a sensitive and specific marker for Leydig cell tumor. It is useful for differential diagnosis of sex-cord stromal tumor (SCST).