A neomycin resistance cassette replaced a genomic fragment containing sequences corresponding to amino acids 211-267, which encode the tripeptide dual phosphorylation motif required for protein kinase activity. Northern blot and RT-PCR analysis on total brain RNA derived from homozygous mice demonstrated that no detectable transcript is produced from this allele. Enzymatic activity assays on extracts derived from hippocampus of homozygous mice confirmed that no functional protein was expressed from this allele.