Direct injection of plasmid DNA into the skeletal muscle has been proposed as a means of effecting somatic gene therapy. We examined the influence of age and sex on the level of expression of an SV40-CAT construct injected into mouse muscle. Age markedly affected expression, with peak values in the 4–6 week age class which were significantly higher than in animals older than 10 weeks. Sex also altered expression, with higher levels of CAT activity seen in males compared to females. We conclude that the rate of growth is important in determining leves of expression of directly injected DNA.
