This study will evaluate the safety and effectiveness of insulin-like growth factor-1 (IGF-1) to treat patients with X-linked severe combined immunodeficiency (XSCID). Those who have XSCID lack white blood cells that protect their bodies from invasion by all types of germs. IGF-1 is the main hormone responsible for the body's growth and metabolism. As a medication, IGF-1 is Increlex[(Trademark)] (mecasermin),