The targeting construct is designed to insert an internal ribosome entry site (IRES), followed by an mCherry fluorescent protein and the codon-improved cre recombinase (icre) sequence separated by a viral T2A oligopeptide that mediates ribosomal skipping, into the 3' UTR. An FRT-flanked neomycin resistance (neo) cassette was inserted 3' of the cre sequence. Flp-mediated recombination removed the FRT-flanked neo cassette