It's a condition so rare, it's not even listed on the National Organization for Rare Disorders.

Chimerism is a rare disorder that happens in the womb where one twin absorbs the other, resulting in two sets of DNA in one person.

This is a story that's making its way around Facebook and Instagram. Mostly because of a young model who recently discovered her birthmark wasn't a birth mark at all -- but the fraternal twin she absorbed in the womb.

Taylor Muhl had battled autoimmune issues her whole life until recently when she went to a doctor who explained the problem was that she had two immune systems.

Muhl's body is constantly working to fight off her twin's cells which her body thinks are harmful to her. As a result, she is constantly battling an autoimmune disease.

Read more about Taylor’s story here.