The cytoskeletal calyx structure surrounds part of the nucleus of the mammalian sperm head and contains two main types of basic proteins: calicin and the multiple band proteins (MBPs). Cylicin-1, a member of the MBP family, contains several lysine dipeptides followed by a third variable amino acid, which, in most cases, is aspartic acid. The central portion of the protein is arranged as a series of repeating units that are predicted to form short individual a-helices which are interrupted by short linker segments. The C-terminal tail of Cylicin-1 contains proline-rich segments. Cylicin-1 is ex-pressed only in the calyx of human and cow spermatozoa, and has a possible architectural role during spermiogenesis.