Mice lacking the UBC4-testis gene have a delay in postnatal testis development but normal spermatogenesis and fertility.
Bedard N, Hingamp P, Pang Z, Karaplis A, Morales C, Trasler J, Cyr D, Gagnon C, Wing SS. Mol Cell Biol. 2005 Aug; 25(15):6346-54.