Pt's dx with Rett's d/o have nomal prenatal and postnatal development but, between ages 5-30 months, they begin to lose previously acquired purposeful hand skills and develop stereotyped hand movements (hand wringing or hand washing) and poorly coordinated gait or trunk movements. They have severe to profound mental retardation and have severe receptive and expressive language deficits. They also lose interest in social interactions. Characteristically, head circumference is normal at birth, but between ages of 5 mo and 4 years the rate of head growth decelerates rapidly. Rett's d/o has been described in females and is very rare.