This study proposes to evaluate the safety and efficacy of Increlex(Trademark) (recombinant human IGF-1) for the treatment of patients with XSCID who have growth failure (children with heights less than 3rd percentile for age). Increlex(Trademark) is a Food and Drug Administration-approved drug for treatment of growth hormone non-responsiveness in the general population of children with growth hormone hyporesponsiveness or primary IGF-1 deficiencies. The scientific objectives are to determine safety and to assess the efficacy of using subcutaneous IGF-1 in XSCID patients with growth failure. The long-term goal of this study is to establish improved treatment regimens for growth failure in children with XSCID.