A selection marker and a transcriptional stop cassette were flanked by loxP sites and placed upstream of a chimeric gene. This chimeric gene encodes the transactivation domains of Tcfe2a (E47) and the bHLH domain of Tal1 (SCL). An IRES-GFP cassette was placed downstream of the chimeric gene and the whole construct (loxP-neo-STOP-loxp- Tcfe2a/Tal1-IRES-GFP) was knocked into the locus. Upon expression of cre recombinase, the chimeric protein and GFP will be expressed, with the chimeric protein predicted to have a dominant negative effect on members of the Id and Tal protein family due to its strong affinity for E transcription factors.