sentences:  Systematic evolution of ligands by exponential enrichment
  
  Systematic evolution of ligands by exponential enrichment (SELEX), also referred to as "in vitro selection" or "in vitro evolution", is a combinatorial chemistry technique in molecular biology for producing oligonucleotides of either single-stranded DNA or RNA that specifically bind to a target ligand or ligands.These single-stranded DNA or RNA are commonly referred to as aptamers.