A loxP site was inserted in exon one 5' of the N-terminal region and a loxP and FRT flanked neo cassette was inserted 3' of exon 1. Cre mediated recombination removed the neo cassette and the N-terminal region of exon 1. Western blot analysis confirmed the absence of expression of the RUNX3(P1) protein that has a distinct set of 19 N-terminal amino acids compared to the RUNX3(P2) protein.