A cassette consisting of a splice acceptor site, an internal ribosomal entry site (IRES), and a lacZ-neomycin resistance gene fusion replaced 2.6 kb of sequence encoding the ATP binding motif P-loop. The cassette was inserted downstream of the ATG-containing exon. A beta-galactosidase fusion protein is expressed from this allele. Immunoblot analysis using a polyclonal antibody against a C-terminal peptide sequence did not detect wild-type protein in brain from homozygous mutant mice.