ROSA26-targeted Flt-1 isoform transgenes partially rescue the ratio of pFlk to total Flk-1. Day 8 differentiated cultures were processed for Western blot analysis with antibodies to total Flk-1 and pFlk (Tyr-1173/1175). (A, top) The signal from hybridization with pFlk antibody. (bottom) Hybridization with a total Flk-1 antibody. All rescue clones tested partially rescued the increased pFlk signal seen in the flt-1^−/− mutant cultures (compare lane 2 with lanes 3–6). (B) Quantitation of the normalized signals from a representative experiment.