A neurological assessment battery will evaluate the following domains: growth, sensory, tone, motor, posture, locomotion, hand use, deep tendon reflexes, abnormal movements, language, and behavior. Participants will be asked to do different kinds of activities such as talking, listening, and drawing, and to play games that involve walking, jumping, concentration, and memory. All participants will be assessed at clinic visits and followed for 72 weeks. AS PER AMENDMENT 10/23/97: A modified version of the neurological battery will be discontinued from interim follow-up evaluations and will be administered one last time as part of the exit evaluation.