Sequence encompassing exons 3 and 4 was replaced with a neomycin selection cassette inserted by homologous recombination. In addition to encoding the helix-hairpin-helix motif necessary for enzyme activty, the deleted region included codon 208, encoding a lysine orthologous to human lysine 212 which is a nucleophile that initiates the formation of a Schiff base ES intermediate. RT-PCR confirmed the absence of normal transcript in homozygous mutant mice. (J:78429)