If the NeoR gene cannot be detected after the first cell infusion, the entire procedure (donor apheresis, gene marking and infusion of cells) will be repeated twice-about once every 6 weeks. If the first infusion was uncomplicated, the second and third infusions may be done on an outpatient basis, with monitoring for 6 hours rather than 24. Six weeks after the third infusion, tests will be scheduled monthly for 6 months and then yearly for long-term follow-up.