1999). We previously described a line of mice [C57BL6-TgN9 (Ins2Cre) 25Mgn] in which a 668 bp promoter fragment of the rat insulin II (RIP2) gene was used to direct Cre expression in mice (Postic et al., 1999; Kulkarni et al., 1999). While insulin promoter fragments of this size are thought to direct expression only to the pancreatic β cell, we detected RIP2-Cre expression in the brain of five different lines of mice, perhaps consistent with reports describing insulin in the brain (Havrankova et al., 1981; Le Roith et al., 1983). The line we further characterize here showed the highest levels of transgene expression within pancreatic β cells and the lowest levels of expression in brain tissue (Postic et al., 1999; data not shown).

Cre-mediated conditional gene inactivation requires precise knowledge of sites of recombination in order to interpret mutant phenotypes. For this reason, we have used ROSA26-stop-lacZ (R26R) reporter mice (Soriano, 1999), which enable recombination to be detected by X-gal staining, to determine sites of RIP2-Cre gene expression during mouse development. Fig. 1A shows a schematic of the RIP2-Cre transgene. In adult mice, recombination within the pancreas was detected exclusively in islets in the majority of core cells (Fig. 1B). This agrees with immunocytochemical detection of RIP2-driven Cre in most pancreatic β cells (Postic et al., 1999). However, we also detected expression within the ventral cerebral cortex (caudate putamen, substantia nigra, hypothalamus, and pons)(Fig. 1C; data not shown) in some but not all neurons (Fig. 1C inset). Glial cells did not express β-gal. In neonates, recombination within the viscera was restricted to insulin-producing islet cells (Fig. 1D–F), as well as ventral brain (data not shown).