Most studies on basic amino-acids-rich CPPs emphasized the importance of the guanidinium side chains of arginines and of the spacing between the charged groups. Studies by Rothbard et al. (18) in particular have shown that a 6-carbon 6-aminohexanoic acid linker seemed optimal for cellular uptake as measured by the whole cell fluorescence but no data concerning efficiency in terms of cytoplasmic or nuclear delivery of a biologically functional payload was provided. We therefore designed a series of (R-X-R) 4 -PMO conjugates with X varying from 2 to 8 carbons (compounds 1-7 in Figure 1A ). The present study revealed a dependence of charge spacing with an optimum for (R-Ahx-R) 4 (in which X = 6) in terms of nuclear delivery of the PMO payload as illustrated below. Based on this first set of data, we designed a series of C6 linked-Arg peptides differing in terms of hydrophobicity (compounds 8-11 in Figure 2A ).