The transgene was designed with an HS4 insulator region, the 3026 bp Ple264 minipromoter (based on NR2E1-F; derived from a portion of the endogenous promoter (Prom 2) and five downstream putative regulatory elements from the human nuclear receptor subfamily 2 (NR2E1) locus), a chimeric intron cassette, the iCre/ERT2 fusion gene (with F3-frt sites flanking the ERT2 sequences), the mut6-WPRE element, an SV40 early polyA signal, an additional HS4 insulator region, a human HPRT complementary sequence (containing exon 1, intron 1, exon 2, and part of intron 2), a mouse 3' Hprt homology arm, an I-Sce linearization cut site, and a mouse 5' Hprt homology arm. The iCre/ERT2 fusion gene used here has an optimized variant of Cre recombinase fused to ERT2; additionally modified to have F3-frt sites flanking the estrogen receptor sequences.