Though never formally diagnosed in his lifetime, Rachmaninoff's exceptional height, freakishly long limbs, big hands, and ridiculous finger-stretch indicate he probably had Marfan's Syndrome. The condition was discovered in 1896 by a French pediatrician, and its hallmarks are usually fairly easy to deduce.

Basically, it's a disorder of the connective tissues that tends to make different aspects of the body stretch, though that is a vast oversimplification from a reporter who knows everything about genetics from the Resident Evil games.

The case for Rachmaninoff having Marfan's goes beyond his physical attributes. He suffered from backaches, arthritis, and eyestrain his whole life... ailments that are typical side effects of the syndrome.

Hell, before he was turned into a blue furball in a laboratory accident, the X-Men's Beast's sole power was basically big hands and agility. Rachmaninoff may not have been able to walk on the ceiling, but then again he wasn't fighting supervillians, he was playing the piano.

Why would he need that particular ability unless he wanted to outdo Jerry Lee Lewis?

To review, Rachmaninoff was born with physical gifts beyond those of regular men through a genetic mutation, suffered personal tragedies that galvanized him into the great man he would become, suffered persecution, and may have used revolutionary mental discipline to further augment his powers.

That screams X-Men more than a lot of others we could mention.