Many of the mammalian sperm surface proteins that have been characterized in recent years have been implicated in sperm-egg interactions and subsequent fertilization. In particular, several members of the MDC protein family, most notably fertilin α, fertilin β and cyritestin (also known as tMDC I), play an important role in sperm-egg interactions in rodents. In light of current data, it is proposed that many of these sperm proteins act co-operatively, and that the concerted effects of several proteins leads to ‘maximum fertilizing ability˚s, but that individual proteins are functionally redundant and are therefore not essential. Nevertheless, the co-operative nature of these interactions means that loss of individual proteins might lead to a concomitant reduction in fertilizing ability, a proposal that is supported by our recent finding of non-functional fertilin α and tMDC I genes in humans.