Btg1-deficiency enhances the proliferative capacity of early FL-HPCs expressing ETV6-RUNX1. FL-derived hematopoietic progenitor cells (FL-HPCs) (cKit⁺Ter119^-) were isolated from wild-type and Btg1^-/- embryos at day 13.5dpc and transduced with control and ETV6-RUNX1 virus. Control and ETV6-RUNX1 transduced FL-HPCs (1x10⁴ cells) were added 48 hours after transduction in B cell specific methylcellulose in the presence of FLT-3L, IL-7 and SCF. Serial replating was performed under identical conditions. Mean colony counts (and SEM) were determined (>30 cells/colony) after 6-10 days of culture. Data is a representative of 2 independent experiments. *, P< 0.05, **, P< 0.01.