Relative luciferase activities of the normal constructs (▪) compared with the mutant constructs (▧). The fold increase is relative to pBtkpro, the construct with no intron 1 sequence. Construct numbers are given below their respective bars: 6, pBtkpro+6; 43, pBtkpro+43; 281, pBtkpro+281; 491, pBtkpro+491; and 1029, pBtkpro+1029. Cell lines are indicated in the top left-hand corner of each series.