While the addition of 10% serum to the medium caused a decrease in the uptake of the R 8 À or R 9 ÀPMOF conjugates, it increased the uptake of conjugates containing RX, RB or RXR motifs (Figure 2A and C) . Serum reduced the MF of R 9 À and R 8 ÀPMOF from 662 and 234 to 354 and 158, respectively, and increased the MF of (RX) 8 À, (RXR) 4 À and (RB) 8 ÀPMOF from 41, 70 and 60 to 92, 92 and 111, respectively. These differences were statistically significant (Figure 2A ). However, higher serum concentrations (30 and 60%) decreased the uptake of (RXR) 4 ÀPMOF and oligoarginineÀPMOF ( Figure 2C ).