Spermatozoa undergo complex sequences of precisely timed events during the process of fertilization. These priming events, which comprise capacitation, egg recognition, acrosome reaction and sperm-oocyte fusion, are regulated by the activation of different intracellular signaling pathways. The efficacy and accuracy of signal transduction pathways often depend on the assembly of multiprotein signaling complexes, thereby coordinating and guiding the flow of regulatory information. To address the question whether PDZ-domain proteins, the most abundant protein interaction modules involved in the assembly of supramolecular signaling complexes, are present in rodent sperm, RT-PCR approaches were performed with specific primer pairs for the vertebrate INAD-like PDZ domain protein MUPP1. The results revealed that this scaffolding protein, which comprises 13 different PDZ domains, is expressed in mouse testis. To obtain further support for the expression of the multi-PDZ domain protein MUPP1 in testicular tissue, immunohistochemical as well as immunocytochemical experiments were performed using a MUPP1-specific antibody. Detailed analyses of the spatial MUPP1-expression profile revealed that immunoreactivity is concentrated within the acrosomal region of round as well as elongated mouse spermatozoa. These results were confirmed in experimental approaches demonstrating that MUPP1 immunofluorescence was shed off from the acrosome region after acrosome reaction. To examine whether MUPP1 is also present in other mammalian sperm, immunocytochemical approaches were performed with isolated bovine, as well as human sperm. The results revealed prominent MUPP1 expression which was restricted to the apical acrosomal region and most notably, to the equatorial segment of the acrosome. The predominant expression profile of MUPP1 in sperm of different mammalian species suggests that this PDZ-domain protein may be involved in organizing signaling molecules mediating primary reactions of fertilization.