E2F1 and Testicular atrophy: In contrast, knockout of E2F-1 produced testicular atrophy [11] with a Sertoli-cell-only (empty tubules) phenotype, and the addition of a hemizygous E2F-3 knockout to a full knockout of E2F-1 accelerates the development of testicular atrophy [12], even though knockout of E2F-3 alone exerts no effect on testis.