An archetypal escort aptamer. (a) A high-affinity aptamer is identified by the SELEX process. 2′-F pyrimidines are incorporated during selections. (b) The aptamer is truncated to minimal size and is now a synthetic molecule. (c) The escort aptamer as an in vivo diagnostic agent. Further nuclease stabilization is achieved: only two positions remain 2′-OH (underlined); the remainder are 2′-F pyrimidine and 2′-OCH₃ purine (bold). Chemical synthesis adds a 3′-3′ exonuclease cap and a primary amine (or thiol, etc.) for desired modifications. For in vivo imaging, a radiometal chelator is conjugated to the 5′ amine and ^99mTc is incorporated.