A loxP site was inserted upstream of exon 2 and an Frt-flanked neo selection cassette as well as a second loxP site were placed downstream of exon 4 of Arntl in a targeting vector. Homologous recombination in ES cells produced a conditional allele of Arntl. The neo was deleted by mating these mutants with transgenic mice expressing flp recombinase. (J:188253)