The editing site complementary sequence (ECS) in intron 11 was replaced with a loxP-flanked neomycin resistance cassette. The neomycin cassette was removed by transient Cre expression in ES cells prior to the production of chimeric mice, leaving a single loxP site in place of the ECS. RT-PCR analysis on RNA derived from brains of homozygous mice demonstrated that unedited transcripts were produced from this allele. (J:30145)