The targeting vector was generated by inserting a codon-optimized intron-CreERT2-NeoR cassette into the endogenous translational start site of Tert in the BAC (RP24-342O18) via recombineering. The targeting vector was linearized and electroporated into JM8/F6 mouse ES cells. Correctly targeted ES clones were selected by Southern blotting and karyotypes, and then injected into ICR/CD-1 blastocysts to generate the knock-in line.