Total 0.1 mg of each (R-X-R) 4 -PMO conjugate were injected in triplicate on a C18 Waters Symmetry Shield 4.6 Â 250 mm column and fitted on a Beckman-Gold HPLC chromatograph. The conjugates were eluted at a flow rate of 1 ml/min of H 2 O/0.1% TFA by a linear gradient of acetonitrile from 5% to 95% in 30 min. Elution of the conjugates was followed by UV absorption at 260 nm. Results are presented as eluting acetonitrile concentrations and expressed as the mean and standard deviation of triplicate measurements.