Figure 2.

tv-a transgenes and RCAS vectors. (A) Transgenes expressing the ALV subgroup A receptor, TVA. The astrocyte-specific Gtv-a transgene was described previously (Holland and Varmus 1998). The Ntv-a transgene utilizes a modified nestin promoter including a portion of its second intron and thymidine kinase promoter sequences that have been shown to direct expression to CNS progenitor cells (Lothian and Lendahl 1997). Transcription start sites are indicated with arrows. (B) RCAS vectors carrying an exogenous gene 3′ of env. RCAS–EGFR* expresses a constitutively active, mutant form of human EGFR (top) with deletions of intra- and extracellular sequences as indicated (TM). The region coding for the transmembrane domain; (δ) the position of the novel junction formed by the deletion in the coding region for the extracellular domain of EGFR. RCAS–AP, RCAS–bFGF, and RCAS–cdk4 carry human placental alkaline phosphatase cDNA, mouse bFGF cDNA, and human cdk4 cDNA, respectively. Although these vectors replicate only in avian cells, the exogenous genes are expressed in both avian and mammalian cells from a spliced message as illustrated.