Effect of strain of mice on correlation of transgene expression levels between reporter gene, as a therapeutic gene, and monitor gene after intramuscular injection of naked pDNA in mice. Symbols and lines express luciferase expression in individual mice and regression lines, respectively. Symbols: Five-week-old male ddY (circles), ICR (triangles), C57BL6 (diamonds) and 10-week-old male ddY mice (squares).