Methylcellulose-based colony-forming assays were plated 72 hours after initiation of transduction by the LN retroviral vector. Cells were transduced in suspension (BSA-coated plates), on stromal support, or on the COOH terminal domain of FN. In all cultures, IL-3, IL-6, and SCF (3/6/S) were used, with or without addition of FLT3 ligand (FL). Colonies were enumerated on day 14. The percentage of G418-resistant colony-forming progenitors was calculated as (no. of G418-resistant CFU/no. of total CFU) × 100. The values shown were obtained from 3,000 CD34⁺ cells plated with and without G418 (0.9 mg/mL).