Purified DNA from a BAC clone (CH211-202A12; BACPAC Resources Center, CHORI, CA) encoding ubb was used as the PCR template. Codon-improved Cre (iCRE) cDNA was PCR amplified from pDIRE (Addgene plasmid #26745; provided by Dr. Rolf Zeller, University of Basel) (Osterwalder et al., 2010), and EGFP cDNA was PCR-amplified using synthetic oligonucleotides with the P2A sequence as the 5′ primer. The components were assembled into pL451 using restriction enzyme digest cloning, and synthetic oligonucleotides with I-SceI (NEB, Ipswich, MA) sites were inserted at the 5′ and 3′ ends of the construct.