Purifying or detecting a target protein (I) in a solution that may also contain a homolog of (I) comprises first contacting the solution with an aptamer that specifically binds to (I) but not to its homolog. An independent claim is also included for selecting an aptamer as above by contacting a mixture of aptamers with the homolog, recovering aptamers that do not bind to the homolog, contacting these aptamers with (I), separating unbound aptamers from aptamer-target complexes, dissociating the complexes, amplifying the dissociated aptamers and repeating the above steps.
