A neomycin cassette replaced exon 1. This exon encodes the initiation site and amino acids 1 - 159 of the protein. Northern blots of brain homogenates from homozygous mutant mice showed no detectable mRNA. Western blots of brain homogenates from homozygous mutant mice showed no detectable protein. However, splice variants are expressed and variable low levels of NOS activity are seen in the brain.