Methylcellulose-based colony-forming assays were plated 96 hours after the initiation of transduction. Supernatant from the PG13 cell line, packaging the LN retroviral vector, was used in all cases. Transduction conditions used were supernatant addition with stromal support (SOS) or supernatant addition in suspension culture with addition of IL-3, IL-6, and SCF (3/6/S) with or without the addition of FLT3 ligand (SNS and SNS + FL, respectively). Colonies were enumerated on day 14. The percentage of G418-resistant colony-forming progenitors was calculated as (no. of G418-resistant CFU/total no. of CFU) × 100. The values shown were obtained from 3,000 CD34⁺ cells plated with and without G418 (0.9 mg/mL).