DMY is the second vertebrate sex-determining gene identified from the fish, Oryzias latipes. Overexpression of genomic DNA carrying DMY in the XX fish induced development of functional testis in these fish. Further, DMY knock-down and estradiol-17b (E2) treatment, to determine the possible function of DMY during early gonadal sex differentiation in XY medaka, revealed that the mitotic and meiotic activities of the germ cells in the 0 day after hatching (dah) DMY knock-down XY larvae were identical to that of the normal XX larvae. Conversely, E2 treatment failed to initiate mitosis in the XY gonad, possibly due to an active DMY, even though it could initiate meiosis.