Transduction of 14-day colony-forming progenitors. CD34⁺ progenitors from six normal human bone marrow samples were transduced in the presence of IL-3, IL-6, and SCF ± FL under the following conditions: BSA, in suspension culture over BSA-coated plates; STRO, on monolayers of irradiated allogeneic stromal cells; FN, on plates coated with the recombinant FN fragment CH-296 (Retronectin). After 72 hours and three additions of supernatant from the PG13/LN cell line, colony-forming assays were plated with and without the selective agent G418. Colonies were counted after 14 to 21 days growth. The percentages of transduction were calculated as (#G418-resistant CFU/# total CFU) × 100 = % G418-resistance.