The actual accumulation mechanism of radiolabeled antisense ODN into GSK1120212 has not been clarified. From the standpoint of antisense mechanism, the difference in stability between double-stranded and single-stranded ODN might be a possible cause of the sequence-specific accumulation of radiolabeled antisense-ODN in target cells. Generally speaking, the accumulation of radiotracer is determined by a combination of uptake, retention and washout at the target tissue. Membrane binding followed by internalization by receptor-mediated fluid-phase pinocytosis and/or adsorptive endocytosis (13) is considered to be the mechanism underlying the uptake of the 111In-MCS-probe. In these processes, however, differences in nucleotide sequences should not play a major role. In addition, both double-stranded and single-stranded ODN are large molecular-weight compounds and it is difficult for them to penetrate the cell membrane for washout. Exocytosis plays a role in the washout of ODN 1 and 13, but it is difficult to believe that it can distinguish single- and double-stranded ODN. Thus, sequence-specific binding itself is not considered to be critical for the target selective accumulation of the 111In-antisense-MCS-probe.