Positive upregulation using the modular Cre/lox system. Floxed effectors can include positive upregulators of the transcriptional activity driving Cre expression. For example, in this model system, Lef-VP16, which upregulates Wnt activity, is expressed from a floxed vector in a Wnt activity-specific manner. The Lef-VP16 feeds back to stimulate Wnt activity, resulting in higher levels of Cre expression and, hence, more production of the floxed effectors, which includes not only the Lef-VP16 itself but also gene products resulting in, for example, cell death. Positive feedback increases the overall efficiency of the system, amplifying the original transcriptional signal, and can be used to enhance the effectiveness of the system under suboptimal conditions, adapted from [33].