The theta domain forebrain enhancer in an AflII-KpnI restriction fragment was replaced by a loxP site flanked neomycin resistance gene cassette. Semi-quantitative RT-PCR and RNA in situ hybridization confirmed the lack of transcript expression in the caudal forebrain primordium at E8.5 but only a reduction in expression at E9.5 and E10.5. (J:163926)