Cylicin-2 is a 348 amino acid protein encoded by the human gene CYLC2. Cylicin-1 is a type of a cytoskeletal protein which contains numerous Lys-Lys-Asp tripeptides accumulated in nine central repetitive units predicted to form a-helices. Cylicin-2, present in cow and human sperm heads, has the same prominent molecular characteristics as Cylicin-1, including a high content of charged amino acids, the abundance of Lys-Lys-Asp tripeptides and repetitive units of presumably a-helical configuration. Cylicin-2 is found in the acrosomal region of round spermatids and in the post-acrosomal region of late spermatids and spermatozoa, in agreement with the localization of Cylicin-1. Cylicin-2 is a novel Actin-binding protein, which probably plays a role in the Actin-related events that occur during spermiogenesis and the early events of fertilization.