Light microscopy of precursor cancer cells in the adult human testis, often referred to as carcinoma-in-situ or CIS. These pre-cancerous cells have large round nuclei with multiple clumps of chromatin, and a pale cytoplasm. Usually found around the inner margin of the affected seminiferous tubule which has no normal germ cells or sperm production. The cause of CIS of the testis is not known. These cells are thought to be abnormal primitive germ cells that arise during fetal development. Magnification x340 when printed at 10 cm.