The endogenous locus was disrupted by homologous recombination of a targeting vector containing sequence encoding a GFP-cre fusion protein followed by an FRT-flanked PGK-neo cassette. While expression of cre recombinase was detected in (and limited to) the megakaryocyte lineage by Western blot analysis, GFP fluorescence was not observed. Endogenous protein was undetected by either flow cytometry or Western blot analysis of homozygous mutant bone marrow cells.