A targeting vector was designed to replace exon 2 with a neomycin resistance gene. This exon was 285-bp long and contained the translation start site and the RII bonding site. Mutants would putatively generate a truncated protein, translated from the next in-frame ATG site in exon 3. Western blot demonstrated that mutants expressed more Mtap2 antigen than wild-type animals. Transcript levels were also increased, suggesting that some modulating regulatory elements are present within the targeted sequence deleted from the locus. (J:110727)