The allele was generated using CRISPR/Cas9 endonuclease-mediated genome editing to have an IRES enhancer sequence and a CreER(T2) fusion gene inserted immediately downstream of the mouse Arg1 gene. These sequences and Cas9 nuclease were introduced into single-cell C57BL/6J zygotes and transferred to pseudopregnant females. Progeny were screened by DNA sequencing to identify correctly targeted pups, which were then bred to C57BL/6J mice for germline transmission.