Exon 3 was replaced with a neomycin selection cassette inserted by homologous recombination. The deleted region encoded amino acids 1 through 288, encompassing the initiating methionine and the first zinc finger domain. Transcript lacking exon 3 was detected in mutant mice by Northern blot analysis of brain tissue. Normal transcript was undetected in the substantia nigra and ventral tegmental area by in situ hybridization. (J:47684)