During early embryogenesis expression is initially detected at the early cleavage stages in the nucleus of two discrete cells. Subsequently, expression is abundant in the cytoplasm of the newly formed pole cells. Male-specific expression during the third larval instar. Mst40 sequences are organized as a tandemly arrayed 1.4 kb repeat unit. The repeat is conserved in all D.melanogaster strains examined but absent from other Drosophila species.