A PGK-neomycin selection cassette replaced part of intron 4 and part of exon 5. RT-PCR analysis on RNA derived from brain of homozygous mice demonstrated that the mutation resulted in the production of a transcript that lacked exon 5 sequences, resulting in a frameshift that introduced a stop codon downstream of the mutation. Western blot analysis on brain lysates from heterozygous mice revealed that a smaller protein of approximately 20 kDa may be produced from this allele.