Scientists from ARIAD Pharmaceuticals (Cambridge, MA) and Stanford University (Stanford, CA) have shown that adeno-associated viral (AAV) vectors significantly outperform adenoviral vectors when used to transfect muscle cells with an inducible gene expression system. In a head-to-head comparison, the scientists used the two vectors to transfect mice muscle cells with a construct containing the gene for human growth hormone (hGH) under the control of a rapamycin-inducible expression system (Proc. Natl. Acad. Sci. USA 96, 8657–8662, 1999). Injection of rapamycin into mouse muscle was shown to rapidly induce hGH synthesis, which gradually declined as rapamycin levels diminished. Importantly, expression of hGH remained stable when AAV was used as vector, but progressively declined when genes were transferred using the adenoviral vector. According to Victor Rivera, corresponding author on the study, AAV integrates its genes into infected host cells and thus avoids immune responses induced by adenovirus that eventually kill cells carrying the foreign gene. "We tried both systems," says Rivera, "and our results demonstrated we should fully explore AAV." He contends that the ease and efficiency of the rapamycin system should make it broadly applicable.