They called it X for unknown

"Yet the X chromosome does much more than help specify an animal’s reproductive plumbing. As scientists who study the chromosome lately have learned, the X is a rich repository of genes vital to brain development and could hold the key to the evolution of our particularly corrugated cortex. Moreover, the X chromosome behaves unlike any of the other chromosomes of the body — unlike little big-man Y, certainly, but also unlike our 22 other pairs of chromosomes, the self-satisfied autosomes that constitute the rest of our genome, of the complete DNA kit packed into every cell that we carry. It is a supple, switchbacking, multitasking gumby doll patch of the genome; and the closer you look, the more Cirque du Soleil it appears." click title for article