Luciferase activity of mutation series vectors. A series of vectors where the sequences from A8 were gradually mutated into 57 sequences were constructed and their luciferase activities were quantified. Mutations from A8 to 57 are indicated by triangles. The mean values from 4–7 independent experiments and the SEM are shown. Statistical comparison was done using the t test. ns: differences were not significant. *: differences were not significant versus R7f-L.