Exon 20 and its two flanking introns were replaced by the corresponding human sequence, containing a T to C transition at position 6 of intron 20 (IVS+6T>C). This mutation is the most common observed in patients with familial dysautonomia. Flp-mediated recombination removed the neomycin resistance cassette inserted into intron 20. QRT-PCR confirmed elevated expression of a transcript skipping exon 20 in the liver and brain. (J:198544)