The gene for the human tMDC I sperm surface protein is non-functional: implications for its proposed role in mammalian sperm-egg recognition

The sperm surface antigen tMDC I (also known as cyritestin) has been proposed to play a role in sperm-egg binding in the mouse via an interaction between its disintegrin-like domain and an integrin receptor on the oolemma plasma membrane. Here we report the cloning and sequence of human tMDC I transcripts and show that they are non-functional, owing to the presence of a variety of deletions, insertions and in-frame termination codons. The absence of a tMDC I protein is further supported by the lack of immunoreactivity on Western blots of human testis and sperm extracts probed with macaque (Macaca fascicularis) and human anti-tMDC I antisera.