Comparison of the effects of different growth factor cocktails on FL CRU self-maintenance in vitro. The number of CRUs recovered from each culture after 2 days (■) and 7 days (□) is expressed as a percent of the input number. CRU numbers were determined by 16-week limiting dilution transplantation assays as described in “Materials and methods.” Values shown are the mean plus or minus the standard error of the mean (SEM) of results pooled from 2 to 5 experiments. Growth factor concentrations are in ng/mL. F indicates FLT-3; 11, IL-11; T, TPO; IGF-2, insulin-like growth factor-2.