Magnesium Supplementation in People With XMEN Syndrome

Background: \- X-linked immunodeficiency with magnesium defect, Epstein-Barr virus infection, and neoplasia syndrome is called XMEN syndrome. In this genetic condition, the cells have less magnesium than normal. This makes it hard for the body to fight infections. Researchers want to see if magnesium supplements can make it easier for the body to fight infection. Objective: \- To see if magnesium supplements can strengthen the immune system and reduce the amount of Epstein-Barr virus in people with XMEN syndrome. Eligibility: \- People ages 6 and older who have XMEN syndrome Design: * Participants will be screened with: * Medical history * Physical exam * CT scan: Participants will drink a contrast and may get dye through an IV in the arm. They will lie in a machine that takes pictures of the body. * EKG: Small sticky patches on the body will trace heart rhythm. * Blood tests * The study has 2 parts. * Participants doing both parts will participate for 1 year and visit the clinic about 15 times. These visits will include a physical exam and blood and urine tests. * Participants doing only the first part finish in 6 months and have fewer visits. * For study part 1, participants will take magnesium pills for 3 months and placebo pills for another 3 months. * At 3 and 6 months, they will have physical exam, medical history, blood and urine tests, and an EKG. * If the magnesium pills are not helpful, participants will do study part 2. * They will be admitted to the hospital for 4 5 days to get magnesium for 3 days through an arm vein. * They will take magnesium pills for another 6 months.