The transgene was designed with an HS4 insulator region, the 2982 bp Ple266 minipromoter (derived from a portion of the endogenous promoter and one upstream putative regulatory element from the human S100 calcium binding protein B - S100B ), 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.