341 nucleotides were deleted from the 3' end of the single exon and replaced with a neomycin selection cassette. The deleted region encoded a portion of the intracellular loop 3, transmembrane domains 6 and 7, and the C-terminus. Absence of ligand-binding in spleen sections and homogenates derived from homozygous mutant mice was determined by receptor autoradiography. In addition, binding studies of brain homogenates confirmed that the brain cannabinoid receptor was unaffected by the targeting event.