The first exon of the Ret gene was targeted for homologous recombination. A sequence encoding inducible cre/ERT2, followed by a loxP-flanked AC-Cre Tn5-neo self-excising cassette, was targeted to exon 1 of Ret in ES cells. Following excision of the floxed neo cassette, a line of knockin mice expressing cre/ERT2 was generated. Spatial and temporal expression patterns of the inducible cre recapitulated Ret expression.