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A promoterless neomycin selection cassette with a splice acceptor site in front of the coding region was inserted into intron 1. The construct was designed to generate a hypomorphic allele, in which the transgenic splice acceptor site competes with an endogenous splice acceptor site in exon 3. Hepatic enzymatic activity, the conversion of porphobilinogen to uroporphyrin, was reduced by 43.4% in heterozygous mutant mice. (J:31572)