A targeting vector was designed to insert human RUNX1 (CBFA2T1), IRES-EGFP and floxed PGK-neo into exon 2 of the endogenous gene. Expression of the inserted mRNA was detectable in peripheral blood leukocytes from heterozygous animals by RT-PCR. Flow cytometry indicated GFP was highly expressed in peripheral blood and bone marrow cells. (J:93408)