A neomycin selection cassette was inserted into exon 6. Gene disruption was confirmed by PCR and Southern blotting. Northern and western blots confirmed that homozygous mutant mice did not show any expression of the enoded RNA or protein. AMPD enzyme activity was almost undetectable in the gastrocnemius and extensor digitorum longus while ~10% of AMPD activity remained in the soleus muscle. Histochemical staining confirmed loss of enzyme activity in all muscle types from homozygous mutant mice.