Rw/Rw+
"Eye colour to be as in standardized variety. To have any standardized coloured body with a white rump. The demarcation line to be straight and circle to body so that the lowest third of the mouse's body will be completely white, inlcuding the back feet and tail. The coloured part of the body to be devoid of any white markings."
Breeding information below the pictures.
Rumpwhite is another dominant pattern, so rumpwhites appear in the first litter when mated for example with a mouse of a 'new' colour. These mice are however often poorly marked and it takes quite some time to get the markings right again. Rumpwhite is lethal when homozygous, but nevertheless rumpwhite to rumpwhite combination works best with the addition of occasional rumpwhite to rumpwhite related self. The things to look for in a rumpwhite are wide area of white with even demarcation line, no white spots on the coloured area, no coloured spots in the white area (both are hard to breed out once they've been bred in) and that the mouse hasn't got a 'zipper' in the belly.
Rw/Rw+ denotes that rumpwhites are heterozygous, with one rumpwhite gene, and the the other allele being 'non-rumpwhite'.