When the different combinations of splitGSK2578215A CreERT2 proteins were analyzed, not all versions showed the same properties regarding these criteria. Generally, the reporter gene expression from split-CreERT2 proteins was lower than that from the non-inducible forms of split-Cre, indicating lower recombination activity of split-CreERT2 Fosfomycin calcium compared to split-Cre in cell culture, which might hamper the efficient targeting of cells in living mice. However, a similarly reduced recombination activity of split-Cre was observed when compared to full-Cre in vitro. Nevertheless, in transgenic mice split-Cre induced robust reporter gene activation, which was probably more limited by mosaic transgene expression rather than by split-Cre efficiency itself. These findings may indicate that the situation in vitro and in vivo differs e.g. in expression kinetics, expression levels, kinetics of activation and DNA recombination. Whether the lower DNA recombination activity of split-CreERT2 in cell culture will limit DNA recombination in mice in vivo remains to be determined experimentally by generating appropriate transgenic mice. The maximal induction ratio observed for split-CreERT2 was about 10, which is in the same range as the reported in vitro induction ratios of CreERT and CreERT2. As both CreERT and CreERT2 have been used successfully in mice in vivo, this suggests that the induction ratios found for split-CreERT2 will be sufficient for successful use in living animals. However, while a high induction ratio is necessary for successful use of split-CreERT2, a high total activity in the presence of 4OHT is also needed. For this reason, we calculated a ����functional index���� as the product of induction ratio and activity. This analysis indicated that the split-CreERT2 variants NE+EC performed best. The lack of recombination activity from the combination of N+CE was unexpected since the fusion of ERT2 to the C-terminal end of CCre is highly comparable to the well established CreERT2 protein. The cloning strategy resulted only in the exchange of three amino acids for two in the region connecting the Cre coding sequence to the ERT2 coding sequence.