In pediatrics clinic, you see a 12 month old girl. As you take a history, you get more and more concerned. The girl was born at term after an uneventful pregnancy and delivery. Both the charts and parent history indicate development was normal for the first 6 months. However, there was deceleration of head growth beginning at 5 months. Today, the parents say the girl has lost developmental milestones. "She woke up and was no longer speaking." There is a loss of acquired fine motor, intellectual, and communicative abilities. As you're talking to the parents, her movements are interesting. Her hands keep moving in a "hand wringing and squeezing" fashion; they seem purposeless. Eventually, this girl may have impaired language, psychomotor retardation, ataxia, growth failure, epilepsy, disorganized breathing pattern, and autonomic nervous system dysfunction.
Challenge: This is a genetic disease only seen in girls. What is it?