The endogenous locus was targeted with a construct containing human cDNA derived from t(8;21), a translocation observed in leukaemic cells of patients with the M2 subtype of acute myeloid leukemia (AML). The human sequence included 62 bp of exon 4 and all of exon 5 of RUNX1 fused to a portion of CBFA2T1. This RUNX1-CBFA2T1 sequence was fused in frame 95 bp into exon 4 of the endogenous murine gene. Hybrid transcript was detected by RT-PCR analysis of differentiated ES cells.