Participants will take sirolimus tablets once a day for 1 year. Three 24-hour urine collections will be done before starting treatment. Blood will be drawn to measure drug levels every week for the first month after starting treatment, then every other week for 1 month, and then every 2 months until treatment stops. Patients who do not have a complete response to the drug at low levels will have their dose increased. Patients will be seen at the NIH clinic in Bethesda, Md., for the screening visit and then at 1, 4, 8, 12, and 15 months for blood and urine tests. Additional urine collections and blood tests will be done periodically throughout the 24-month study period by the patient's local physician.