This protocol studies carrier testing decisions of female adolescents who have a male relative with X-linked severe combined immunodeficiency (XSCID). XSCID is a rare immunodeficiency that affects only males, although females carrying XSCID mutations can pass the disease to their male offspring. In previous generations infant boys with XSCID uniformly died, but medical interventions such as bone marrow transplantation and now gene therapy, have proven life-saving for many patients. XSCID was mapped in 1987, and the disease gene was identified in 1993; these advances made possible patient and carrier diagnosis, which have been available on a research basis for 10 years through IRB-approved protocols of Dr. Puck. A large sample of XSCID families enrolling in molecular diagnosis protocols participated in her psychological study of the impact on adult siblings of having a brother affected with XSCID (Protocol OH98-HG-N015, Perception of Carrier Status in Families with X-Linked Severe Combined Immunodeficiency, now complete and terminated). That study indicated strong interest on the part of at-risk females for carrier diagnosis before reaching the age of adulthood. The current protocol is a follow-up protocol that functions in concert with Protocol 95-HG-0066 (Genetic Analysis of Immune Disorders) to study the decision-making process and the impact of carrier testing in female adolescents from families with known XSCID mutations. With parental consent, carrier testing for girls from age 13 through age 17 will be discussed in a genetic counseling session. The 50% risk of being a carrier and the 50% chance of XSCID in male offspring of carriers will be explained. Possible psychological and other benefits and harms of testing will be aired. If elected by the girl, XSCID mutation testing will be performed by having her enroll in our genetic testing protocol 95-HG-0066. Follow-up interviews with all girls enrolled, both tested and not tested, will be conducted at 3 months and a year post encounter.