Port wine stain genetic switch
Researchers found a gene with a molecular switch locked permanently 'on' that is believed to cause port wine stain birthmarks.
Snapshot · Stuff You Should Know
Selects: Birthmarks: Probably Not the Mark of the Devil
Explore episode
Jun 20, 2026
Where this was said
Pigmented Birthmarks: Café au Lait and the Melanin Tangent
At 21:40 · chapter starts 19:45
Josh and Chuck pivot to the second major category: pigmented birthmarks caused by melanin concentration rather than blood vessels. The name-dropping of café au lait birthmarks sparks a genuine tangent about what café au lait actually means (coffee with milk), which leads to a comprehensive breakdown of espresso drinks, which leads to a discussion of sushi rice technique involving rice vinegar cooked in from the start, which leads to the DeKalb Farmers Market and eel. It's peak Stuff You Should Know digression. Sandwiched in the middle is a genuinely touching moment: Chuck describes talking to his young daughter about skin color for the first time, explaining melanin scientifically as a way to frame race as natural variation rather than hierarchy.
Port Wine Stains: The Birthmark With a Known Genetic Clue
Port wine stains are the only birthmark type with a partially identified genetic cause: a molecular switch permanently stuck in the 'on' position. About 6% of cases are linked to Sturge-Weber syndrome, a neurological condition — and 100% of Sturge-Weber patients have the port wine stain.
Science can't explain most birthmarks
According to dermatologists, the medical community still does not know what causes most birthmarks; port wine stains are the closest to having a known genetic cause.
Port wine stain & Sturge-Weber link
About 6% of children born with a port wine stain birthmark develop Sturge-Weber syndrome, a neurological condition — but 100% of Sturge-Weber patients have the port wine stain.
Café au Lait Spots and the 6-Mole Warning
Café au lait birthmarks — light brown spots that can appear anywhere on the body — are usually harmless. But having 6 or more, especially larger than a dime, may indicate neurofibromatosis (NF1), a nerve condition. It's not a death sentence, but it is a sign to see a doctor.
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