We attempted to understand the nature of (RX) 8 À and (RXR) 4 ÀPMO toxicity. It is apparent that these two conjugates caused little immediate membrane damage with 0.5 or 5 h treatment at concentrations as high as 60 mM (Figure 8 ). However, these two conjugates had dose-dependent toxicity with 24 hr treatment as shown by the leaky cell membranes and fewer cells compared to controls ( Figure 6C&D, Figure 8 ). Interestingly, the replacement of Xs with Bs in (RX) 8 ÀPMO abolished the toxicity, and the replacement of L-arginine with D-arginine reduced the toxicity of (RX) 8 ÀPMO ( Figure 6 ). We have found that (rX) 8 ÀPMO was completely stable and the peptide portion of (RB) 8 ÀPMO was only partially degraded, whereas the peptide portion of (RX) 8 ÀPMO was completely degraded in cells (5) . We wondered whether the difference in toxicity among (RX) 8 , (RB) 8 and (rX) 8 ÀPMO conjugates was caused by differences in intracellular stability, resulting in the metabolized products of (RX) 8 ÀPMO producing toxicity. The identifiable metabolized products of (RX) 8 ÀPMO were XRXBÀPMO and XBÀPMO (5) but neither product had any detectable toxicity as measured by MTT assay (data not shown). It is possible that the CPP portion was degraded into free amino acids and/or smaller peptide fragments which were toxic. However, our investigation revealed that neither free R nor X, alone or in combination, caused cellular toxicity. Another possibility is that because of the high hydrophobicity of X compared to B, X in combination with positively charged arginine residues leads to toxicity not generated by B residue combinations. However, this explanation does not account for the difference in toxicity observed between (RX) 8 ÀPMO and (rX) 8 ÀPMO, which have the same hydrophobicity. Perhaps the toxicity of (RXR) 4 ÀPMO and (RX) 8 ÀPMO was caused by the peptide fragments that we could not identify by mass spectrometry.