The W series is referring to mutations on the KIT locus known as Dominant White, White, or White Spotting. There are now 21 genetically discovered mutations for the W series. W21 is found in an Icelandic horse with a Sabino-like phenotype. The Icelandic tested negative for Frame and Splash white and tested heterozygous for the new W21 mutation. The horse and the study can be seen here.
There are many known mutations on the KIT locus, including Tobaino, Classic Roan, Sabino 1 and W1-21.
The W series currently: