Donors will undergo the apheresis procedures required by the NIH protocol they are enrolled in. Throughout the procedure, they will receive an intravenous infusion of a salt solution that may or may not contain magnesium. Blood samples of 5 milliliters (1 teaspoon) each will be taken from the apheresis machine at the beginning and end of the procedure and at 30- to 60-minute intervals during the procedure. No more than 50 ml (3 tablespoons) will be taken during any single apheresis. The last sample will be drawn 60 minutes after completion of the apheresis. In addition, donors will: