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An FRT-flanked neomycin resistance cassette with a 5' loxP site was inserted upstream of exon 2. An additional loxP site was inserted in exon 2 downstream of the coding sequence. Cre-mediated recombination removed the loxP-flanked region, including the coding sequence and the selection cassette. RT-PCR analysis revealed absence of RNA expression in the liver, whole brain, and skeletal muscle of homozygous mutant mice. Absence of serum levels in chow-fed conditions was confirmed by ELISA. (J:225402)