A vector was inserted in exon 3 that replaces the promoter, exon 1, and exon 2 that are missing in the endogenous gene in ES cell line E14TG2a. Correct targeting was expected to result in a duplicate copy of exon 3 located 5' of the promoter. In the specific clone used this duplicated exon was deleted resulting in restoration of the functional gene structure. Northern blot analysis confirmed that the corrected transcript was produced.