Cre recombinase is a classic tool for in vivo conditional mutagenesis.In transgenic mice, a Cre "driver" line provides tissue and temporal specificity of recombination using regulatory sequences to precisely control Cre expression.Cre driver mice thus allow conditional ablation or activation of genes when crossed with mouse lines in which loxP sites flank either endogenous genes or transgene expression cassettes.One major obstacle for conditional mutagenesis is the limited availability of truly cell type-specific promoters as many subsets of cells can only be unambiguously defined by a combination of several marker genes.