Sertoli cell-only syndrome (SCO syndrome) is a cause of male infertility. In SCO syndrome, only Sertoli cells (cells that nurture the immature sperm) line the seminiferous tubules (tubes inside the testicles where sperm develop). Therefore, there are not any sperm cells present in the seminiferous tubules. Men typically learn they are affected when being evaluated for infertility and are found to have no sperm production (azoospermia). Other signs and symptoms are rare, but in some cases there could be an underlying cause of SCO syndrome that causes other symptoms, such as Klinefelter syndrome. Most cases of SCO syndrome are idiopathic (of unknown cause), but causes may include deletions of genetic information on regions of the Y-chromosome, especially on the azoospermia factor (AZF) region of Y-chromosome. Other causes include exposure to chemicals or toxins, history of radiation therapy, and history of severe trauma. Diagnosis of SCO syndrome is confirmed with testicular biopsy.