It is well known that cytoskeleton and karyoskeleton proteins are associated with changes in cell shape and with the rearrangement of the dynamic structures involved in cell division and motility. In higher vertebrates, there are three major skeletal protein groups: microfilaments, microtubules and intermediate filaments, each representing a multigene family. Some of these skeletal proteins are expressed in a temporally- and spatially-specific fashion, and they establish cell-specific cytoplasmic and nucleoplasmic organization during development. Here we report the cDNA cloning of a novel 60 kDa skeletal protein from mouse spermatocytes, termed MNS 1 (meiosis-specific nuclear structural protein), whose computer-predicted protein configuration indicates long alpha-helical coiled-coil domains flanked by non-helical terminal domains. Functional characterization of MNS1 by ectopic expression in culture cells indicated that it is a detergent- and high salt-resistant skeletal protein which is involved in organization of the nuclear or perinuclear architecture. The MNS1 protein is specifically expressed at the pachytene stage during spermatogenesis, so that its function may involve the determination and maintenance of the appropriate nuclear morphology during meiotic prophase.