It's a gene, somehere, somehow; by now there are probablly quite a number of transgenic combos that have showed up in the pet and fancy mouse supply. I've been working with a line that carries something like this, but not 'zackly like this. On top of every other weird thing about transgenic meeces, there's a type that produces hermaphrodites(XXY), and a type that translocates portions of the chromosome 7 that affect the c-locus and a-locus, and whether any or all of these qualities are present in any given odd unclassifiable mousie is anybody's guess. Someone has yet to take a look in the genes we find in the mouse fancy and attribute the correct genotypes to the phenotypes.
(They're Pinky, they're Pinky, they're Pinky and the Brain, Brain, Brain, Brain....)