IN VIVO SELECTION WITH O6-BENZYLGUANINE AND CARMUSTINE: Between 28-180 days after infusion of gene-modified HSPC and after hematopoietic recovery from first line treatment, patients with >= 1% and < 10% gene marked cells receive O6-benzylguanine IV over 1 hour, immediately followed by carmustine IV, and followed 7-8 hours later by O6-benzylguanine IV over 1 hour. Treatment continues for an additional course provided that gene marked cells does not meet or exceed 10% of peripheral blood cells and in the absence of unacceptable toxicity.