GFAP-HSV-TK transgene. The transgene consists of the human glial fibrillary acidic protein gfa2 promoter joined to the HSV-TK gene. The gfa2 fragment spans bp −2163 to +47 relative to the transcriptional start site, with the natural-initiating ATG at bp +15 converted to TTG by site-directed mutagenesis so that protein translation initiates within the TK. Thefilled-in portion of thebox corresponds to the TK coding region. The Gfa2-TK2 transgene is essentially identical, except that the 3′ flanking region of the TK gene has been replaced by part of the mouse protamine-1 gene, which supplies an intron and polyadenylation site.