The deduced 1,265-amino acid protein shares significant similarity with mouse Mast205. RT-PCR ELISA detected variable MAST2 expression in all tissues examined. The deduced 1,734-amino acid protein has a calculated molecular mass of 190.5 kD. It had an apparent molecular mass of 205 kD by SDS-PAGE. Mast205 contains a central serine/threonine kinase catalytic domain, which includes an ATP-binding consensus sequence, and several potential phosphorylation sites. Northern blot analysis of mouse tissues detected Mast205 expression only in testis. Mast205 expression increased during postnatal testicular development coincident with meiosis/spermiogenesis. Immunofluorescence microscopy showed Mast205 associated with mouse spermatid manchette.