A port-wine stain is a vascular birthmark consisting of superficial and deep dilated capillaries in the skin which produce a reddish to purplish discolouration of the skin.
A port-wine stain happens when chemical signals in tiny blood vessels don't "turn off," and those blood vessels get bigger. The extra blood turns the skin red. Researchers found that port-wine stains start with the mutation of one specific gene. What causes this change in a developing baby's DNA still isn't clear.