C/EBPα binding activities in FL extracts and C/EBPα^-/- FL cell lines and transplantation of C/EBPα^-/- FL cells in vivo. (A) Nuclear extracts prepared from wild-type FL (left panel), mutant FL (middle), or the C/EBPα^-/- FL cell line cultured in IL-3 (right) were analyzed by EMSA using a consensus C/EBPα-binding site probe. Binding reactions included either nonimmune (NI) serum or the indicated C/EBPα antibodies. DNA-protein complexes were resolved on nondenaturing polyacrylamide gels. The identities of various dimeric C/EBPα species are shown by arrows; asterisks denote complexes that are supershifted by the antibody. (B) C/EBPα^+/+ and C/EBPα^-/- FL cells were transplanted into lethally irradiated mouse recipients at the indicated cell doses and monitored for survival according to the procedures described in “Materials and methods.” (C) In other groups, spleens were removed 10 to 12 days after transplantation and fixed in Tellesniczky solution to visualize spleen colonies (CFU-Ss) and photographed.