Number and position of X residues. Having shown that (RB) 8 ÀPMO had less activity than (RXR) 4 ÀPMO or (RX) 8 ÀPMO, we further investigated the effect of the number and position of X residues on the activity of conjugates. Eleven CPPÀPMO conjugates containing 0, 2, 3, 4, 5 or 8 Xs were compared (Figure 4) . Generally, CPPs containing a higher number of Xs had higher activities. At 2 mM, (RX) 8 ÀPMO (8 X residues) had the highest activity followed by (RXR) 4 ÀPMO (5 X residues) and the conjugates with fewer Xs had lower activities. At 5 mM, three conjugates containing 3 (3d), 4 (4c) and 8 ((RX) 8 ) X residues had the highest activities, suggesting that the position of X residues affects activity.