A cellular IRES in the 5' UTR of exon 3, the coding region of exon 3 and part of intron 3 were replaced with a floxed neo cassette and a Runx1 and Mecom fusion cDNA via homologous recombination. The neo cassette was removed by cre mediated recombination. RT_PCR confirmed the expression of the fusion gene from both endogenous promoters. Western blot analysis confirmed expression of the fusion protein. (J:164806)