A targeting vector was used to place a loxP site in the 5' UTR of the gene in the AflII restriction site 70 bp 5' of the ATG. A tetracysteine tag was inserted in frame into the first coding exon (from GGLSGSS to GGLSGCCPGCCSGSS) and an frt-neo-frt-loxp cassette was inserted 110 bp downstream of exon 1 into the genomic NdeI site. The line was crossed to cre-transgenic mice to excise the first coding exon. Total brain protein levels are normal. Rims1 is 30% more soluble in a detergent-free fractionation.