The targeting construct was generated by inserting the diphtheria toxin A gene (DTA, for positive selection), 3.2 kb 5 homologous arm containing the 5' UTR of exon 1, and 5.5 kb 3 homologous arm containing exon 2 into pBluescript SKII(+). Then the eGFP-CreERT2 (GCE) fragment (McMahon et al. 2008) with an SV40 poly adenylation site and neomycin resistance cassette were inserted immediately downstream of the translational initiation codon ATG. The knock-in construct removed the coding sequences from exon 1 and placed the GCE gene under the control of endogenous regulatory sequences. Flp-mediated recombination removed the selection cassette. The EGFP could not be detected by direct fluorescence or anti-GFP antibody staining.