CXCL12 and cancer: In the treatment of cancer, aptamers can serve as free molecules targeting specific cancer biomarkers as either agonists or antagonists (Fig. 5).375 Both AS1411 and NOX-A12, which have reached the clinical trial stage, are antagonistic aptamers that specifically target nucleolin and CXCL12, respectively.376–379 Nucleolin is highly expressed in multiple tumor cells.