Luo, L. et al. Neuron https://doi.org/10.1016/j.neuron.2020.01.008 (2020).

The Cre–loxP system allows temporal and/or spatial control over reporter gene expression in model organisms, particularly the mouse. But all is not well with this tool: undesirable expression patterns can appear. Such problems can arise from unwanted germline expression of the Cre recombinase. Alternatively, mosaic expression of the Cre driver can complicate genotyping. In addition, recombination can be more or less efficient, depending on the floxed locus. Luo et al. have analyzed the rates of germline recombination in two commonly used Cre mouse lines and collected similar data for a broad selection of additional Cre lines widely used in the neuroscience community. The researchers recommend breeding strategies to deal with these problems and provide additional guidelines. Finally, the phenomenon is not restricted to mice but can also be observed in zebrafish Cre lines.