A neomycin resistance cassette replaced part of the exon 1 coding sequence (bp 298 - 381) into ~1kb of intron 1. Northern blots of brain homogenates from homozygous mutant mice showed minimally detectable truncated Aqp4 transcripts. Western blots of brain homogenates from homozygous mutant mice showed no detectable Aqp4 protein. Immunocytochemistry of renal tissue from homozygous mutant mice showed no detectable Aqp4 protein.