Many research studies use immortalized cell lines as surrogates for primary beta- cells. We describe the production and use of a novel "indirect" dual-fluorescent reporter system that leads to mutually exclusive expression of EGFP in insulin-producing (INS(+)) beta-cells or mCherry in non-beta-cells. Our system uses the human insulin promoter to initiate a Cre-mediated shift in reporter color within a single transgene construct and is useful for FACS selection of cells from single cultures for further analysis. Application of our reporter to presumably clonal HIT-T15 insulinoma cells, as well as other presumably clonal lines, indicates that these cultures are in fact heterogeneous with respect to INS(+) phenotype. Our strategy could be easily applied to other cell- or tissue-specific promoters. We anticipate its utility for FACS purification of INS(+) and glucose-responsive beta-like-cells from primary human islet cell isolates or in vitro differentiated pluripotent stem cells.