aptamer

An aptamer is a short strand of DNA or RNA that folds up into a precise three-dimensional shape and grips a target the way an antibody does. Most nucleic acids are thought of as flat strings of information, but an aptamer crumples into a specific knot whose contours wrap around a protein, a small molecule, or even a cell-surface feature.

Aptamers are not designed by reading a complementary sequence; instead they are selected from huge random pools through a repeated process called SELEX, in which strands that stick to the target are pulled out and amplified, round after round, until only tight binders remain. The winning sequence is then synthesized chemically, like a small-molecule drug rather than grown in cells.

This makes aptamers an interesting middle path: they bind with antibody-like specificity but are made by chemistry, are non-immunogenic, and are cheaper and easier to modify. Their weakness, like other oligonucleotides, is that natural versions are quickly degraded and cleared, so therapeutic aptamers carry chemical modifications, and relatively few have reached the clinic so far.