Frank Lentini was born with a condition known as a parasitic twin, also referred to as heteropagus twinning. This condition occurs when one twin stops developing during gestation and becomes vestigial to the fully formed twin. The fully formed twin, in this case Lentini, can end up with extra limbs or organs. In Lentini’s case, he was born with three legs, four feet, 16 fingers, and two sets of genitals. His extra leg sprouted from the side of his right hip, with a fourth foot protruding from his knee. This condition is extremely rare, occurring in as few as one in a million births.