Fig. 1. Oligonucleotide uptake and trafficking in cells. It has been proposed that phosphonothioate antisense oligonucleotides (PS-ASOs) enter cells via two pathways: productive pathway and non-productive pathway. Productive pathway guides PS-ASOs to access their targets (yellow outlines), and non-productive pathways sequester PS-ASOs in saturable sinks (blue outlines). Uptake and internalization of PS-ASOs can be directed by cell-surface proteins (including receptors) through caveolin- or clathrin-dependent pathways or other pathways of endocytosis. The endocytosis vesicle bud from the membrane and the scission of the plasma membrane is catalyzed by GTPase dynamin. Freed cargo-containing vesicles with PS-ASOs fuse with early endosomes (EE) by being transported along the cytoskeletal structures of tubulin or actin. Internalized ASOs can be transported from EE to late endosomes (LE) and to lysosomes. PS-ASOs predominately accumulate in punctate structures in EE, LE, lysosomes (possibly equal to phosphonothioate bodies of cytoplasm), and Golgi-58K-related vesicles, in which non-productive and productive pathways of uptake may diverge and the majority of oligonucleotides are likely reserved in non-productive pathway. A small number of PS-ASOs can be released from endosomal organelles and escape into productive pathway, involving entry into the nucleus and cytosol to act on target DNA. Entering into the nucleus is likely via the RAN-mediated pathway. PS-ASOs may be linked to paraspeckle proteins in the nucleus and shape functional paraspeckle-like structures.