It is a common coat mutation with the mutant allele of U.
Oter examples:
a Melanistic black
b Rust, (Guinea Gold)
Ba Banded, (White Banded)
cd Acromelanic gene - "Dark Eared White"
dg Dark Grey
Ds Dominant Spotting
e Cream
Lg Light Grey, (Lethal Grey)
l Longhaired/ Angora/ Teddy Bear
p Cinnamon
ru Ruby Eyed Fawn
rx Rex coated
s Spotting, (Piebald)
Sg Silver Grey
To Yellow, (sex linked, see later)
U Umbrous
Wh Anopthalmic, (Eyeless), White, (White bellied)
Source:
Genetics and Inheritance