A neomycin resistance cassette replaced 300 bp of genomic DNA including exon 23a. RT-PCR analysis of brain detected neurofibromin type I but not the type II isoform in homozygous mutant mice. Western blot analysis detected the type I but not the type II neurofibronin in brain of homozygous mutant mice. Immunohistochemical analysis of brain did not detect type II neurofibromin isoform in homozygous mutant mice.