Systematic evolution of ligands by exponential enrichment (SELEX) technology has been under development for a decade, and is a general method for identification of oligonucleotide ligands as potential drugs. A pool of randomized RNA or single stranded DNA sequences were selected against certain targets. The sequences of tighter binding with the targets were isolated and amplified. The selection was repeated using the enriched pool derived from the first round selection. Several rounds of this process lead to winning sequences that are called 'aptamers' or 'ligands'. This technology has been applied to a wide range of targets including various enzymes of HIV, growth factors and inflammation-inducing enzymes. The first aptamer that has proceeded to phase I clinical studies is NX-1838, an injectable angiogenesis inhibitor that can be potentially used to treat macular degeneration-induced blindness.