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Traditionally people thought that cells didn't make it to the front of embryos to pigment the belly because they just weren't migrating fast enough, what we've been able to show through our studies is that actually, if anything, cells in piebald animals migrate faster but they're just not proliferating enough. They're not making enough daughter cells to colonize - or cover - the whole region of the skin that needs to be covered by the time the pigmentation pattern is set down.