Soulmates

"Sherlock has had his marks as long as he can remember. When he was a baby his fingers were bright blue, when he grew older his hands became blue too.
Everyone has soulmate marks. Mummy has golden lines on her back and dad has deep scarlet lines. Mycroft has grey spots covering his chest and neck. At first it was just his chest but soulmate marks grow until you meet your soulmate."