Three micrograms of each (R-X-R) 4 -PMO conjugate were injected in triplicate on a 1 ml HiTrap Sepharose/heparin column (Amersham Biosciences, Freiburg, Germany) fitted on a Beckman-Gold HPLC chromatograph (Beckman Coulter, Fullerton, CA, USA). The conjugates were eluted in 30 min at a flow rate of 1 ml/min of 2.5 mM phosphate buffer (pH 7) by a linear gradient of NaCl from 70 to 970 mM. Elution of the conjugates was followed by UV absorption at 260 nm. Results are presented as eluting NaCl concentrations and expressed as the mean and standard deviation of triplicate measurements.