Exon 5 was replaced by a construct composed of the first 25 bp of mouse exon 5 fused to the 3' end of a human CBFB/MYH11 cDNA. The human cDNA fragment used contained the last 66 bp of human exon 5 and exons 33-42 of MYH11. The sequence in exon 33 encoding amino acids 179-221 are deleted. A bovine growth hormone polyadenylation site and a floxed PGK-neomycin cassette were inserted immediately downstream.