Butt dot?

Dear M.A.:

Getting personal, I have a small blue dot on my butt, and I'm told my mother and brother do as well. My mother has always said it was a "Mongolian birthmark." She says that people of Mongolian descent (I'm half Chinese) are born with such a mark. Is this possibly true, or is she pulling my leg? What is the truth, and why is it there?

-- Berry Bottom, San Diego

Well now you've gone and hurt Mom's feelings. Why would she lie to you about something so important? Your butt bears what's variously known in the medical biz as a Mogolian spot, a nevus fuscocaerulius, blue spot, sacral spot, Mongolian macula, nevus of Ito, or nevus of Ota. It's a bluish-gray accumulation of cells present at birth on the lower back or buttocks of many Asian, black or other dark-skinned people. It usually vanishes by the time a child is five years old. As far as I know, the spot is utterly without meaning or purpose, and the fact that yours hasn't disappeared is just the luck of the genetic draw. But I'm sure it makes for a dandy ice breaker at cocktail parties.