In recent years, much knowledge about the functions of defined genes in spermatogenesis has been gained by making use of mouse transgenic and gene knockout models. Single null mutations in mouse genes encoding four male germ cell proteins, transition protein 2 (Tnp-2), proacrosin (Acr), histone H1.1 (H1.1), and histone H1t (H1t), have been generated and analyzed. Tnp-2 is believed to participate in the removal of the nuclear histones and initial condensation of the spermatid nucleus. Proacrosin is an acrosomal protease synthesized as a proenzyme and activated into acrosin during the acrosome reaction. The linker histone subtype H1.1 belongs to the group of main-type histones and is synthesized in somatic tissues and germ cells during the S-phase of the cell cycle. The histone gene H1t is expressed exclusively in spermatocytes and may have a function in establishing an open chromatin structure for the replacement of histones by transition proteins and protamines. Male mutant mice lacking any of these proteins show no apparent defects in spermatogenesis or fertility. To examine the synergistic effects of these proteins in spermatogenesis and during fertilization, two lines of triple null mice (Tnp-2-/-/Acr-/-/H1.1-/- and Tnp-2-/-/Acr-/-/H1t-/-) were established. Both lines are fertile and show normal sperm parameters, which clearly demonstrate the functional redundancy of these proteins in male mouse fertility. However, sperm only deficient for Acr (Acr-/-) are able to compete significantly with sperm from triple knockout mice Tnp-2-/-/Acr-/-/H1.1-/- (70.7% vs. 29.3%) but not with sperm from triple knockout mice Tnp-2-/-/Acr-/-/H1t-/- (53.6% vs. 46.4%). These results are consistent with a model that suggests that some sperm proteins play a role during sperm competition.