A targeting construct was generated by homologous recombination in a BAC containing the Neurog2 gene. An ERT2iCreERT2 sequence (icre/ERT2*; codon inproved cre fused in-frame N- and C-terminally with mutated human estrogen receptors) was inserted into exon 2 of Neurog2, replacing the entire coding sequence. The construct was electroporated into ES cells for homologous recombination.