Mice homozygous for a knock-out allele exhibit male infertility associated with small testes, meiotic failure, increased male germ cell apoptosis, and azoospermia. Male homozygotes for a different knock-out allele show impaired spermatogenesis, an expanded testicular interstitium, seminiferous tubule atrophy, increased Leydig and Sertoli cell numbers, and enhanced testosterone synthesis along with cellular senescence in Leydig cells.