A9g aptamer (red helices) bound to a prostate-specific membrane antigen (PSMA) molecule. Aptamers are short sequences of artificial nucleic acid, in this case RNA (ribonucleic acid), that bind to a specific molecule. Aptamer A9g binds to PSMA, also known as glutamate carboxypeptidase II, a tumour marker associated with prostate cancer. Therefore A9g has applications in the imaging of cancerous cells and drug delivery to them.