PTK7 and leukemia: Through Systematic Evolution of Ligands by Exponential Enrichment (SELEX), nucleic acid aptamers can be evolved to bind a protein target with high affinity and specificity (Fig. 3d).86,87 Protein tyrosine kinase 7 (PTK7), a cell–surface protein overexpressed in subtypes of leukemia, could be recognized by an aptamer hybridized with a cy3-labeled oligonucleotide.