“…Xmrk-2 has been mapped to the sex-chromosomal region harboring the macromelanophore locus (Schartl, 1990;Weis and Schartl, 1998). Both the macromelanophore locus and Xmrk-2 are considered`accessory' loci, as individual ®sh within a population can be found without either locus (Schartl, 1990;Weis and Schartl, 1998;Wittbrodt et al, 1989;Woolcock et al, 1994;Zechel et al, 1989). A structurally closely related gene known as Xmrk-1 also resides on the sex chromosomes, but its expression is not correlated to melanomagenesis; instead, it is expressed during embryogenesis, and in adult tissues as well (Adam et al, 1991;Woolcock et al, 1994;Zechel et al, 1992a,b).…”