A simple FL-dependent proliferation and immortalization cell system can evaluate cooperation between Meis1 and Hoxa9. (A) Growth curve of progenitors cultured in FL following infection with empty vector (▵), Hoxa9 (□), Meis1 (○), or Meis1 plus Hoxa9 (▪). Error bar indicates standard deviation of data from 3 repeated experiments. (B) Anti-FLT3 immunoblot demonstrating strong FLT3 expression in representative leukemias arising from Hoxa9/Meis1-immortalized progenitors derived in either SCF or FL. Immunoblots were performed on cell extracts from a control cell line (C), from the bone marrow (M), spleen (S), and lymph nodes (N) of leukemic mice, and from cultured leukemic blasts from splenic tissue (L).