HGG Community Forums

It is caused by KP

When there are two KIT mutations present and two copies of KP a cascade failure in pigment can occur resulting in completely random white markings. Size is influenced by white factor.

This page has a lot of info about what various genes do
https://www.huntandjump.com/geneticsguide/

Kit promoter does this roaning effect. Kit promoter 2 does this but also creates diffuse edges on the markings that exist.
Also, kit promoter 1 requires 2 kit mutations and 2 copies of kp.
Kit promoter 2 has an effect on just 1 kit mutation and with just 1 copy of kp2. If a horse has 2 kit mutations and is hom kp2, it's the most dramatic roaning/whitening.