To determine the effect of serum on cellular uptake of the various conjugates, uptake evaluation assays were carried out in the medium containing various concentrations of, or in the absence of, serum. Cellular uptake of CPPÀPMOF conjugates increased with the number of arginines and decreased with the X and/or B residue insertion (Figure 2A and B). The oligoarginine R 9 ÀPMOF had a mean fluorescence (MF) of 662, nearly 3-fold higher than the 234 produced by R 8 ÀPMOF, indicating that a difference of a single arginine can make a substantial difference in the biological properties of a CPP. Insertion of an X or B residue in the R 8 sequence reduced the MF from 234 of R 8 ÀPMO to 42, 70 and 60 of (RX) 8 À, (RXR) 4 À and (RB) 8 ÀPMOF, respectively ( Figure 2A ). The number of RX or RB repeats affected cellular uptake, with conjugates having fewer RX or RB repeats generating lower MF ( Figure 2B) .