<n0:ComplexDefinition xmlns:n0="http://ncicb.nci.nih.gov/xml/owl/EVS/ComplexProperties.xsd#"><n0:def-definition>Double-stranded DNA or single-stranded RNA oligonucleotide sequences with the capacity to recognize target molecules with high affinity and specificity. Fundamentally distinct from antibodies, aptamers mimic their properties and have emerged as a promising class of molecules in both therapeutic and diagnostic formats. In having small size (8-15 kDa) relative to antibodies (150 kDa), aptamers are candidates for rapid tumor penetration and blood clearance.</n0:def-definition><n0:def-source>NCI</n0:def-source></n0:ComplexDefinition>