Many protein assays and immunoassays are built around antibodies. Sometimes the antibody is the part you cannot get. The target is too similar across species to raise a useful immune response. Or it holds the relevant shape only in a membrane and loses it during purification. Or antibodies exist, but no validated pair does.

The format itself is ordinary. A sandwich ELISA, a bead assay, a lateral-flow strip. Thousands of related assays have been run and detailed protocols exist – just not for your target.

An aptamer is one route around the missing antibody: a short DNA or RNA sequence that folds into a structure capable of binding a target. It is not a small antibody, and it cannot be dropped into an antibody protocol unchanged. But the protocol still carries most of what it cost to develop. And it carries more than the assay.