Protocol for purification of synthetic oligonucleotides by polyacrylamide gel electrophoresis. As a rule of thumb, oligonucleotides >25 nucleotides should be purified by polyacrylamide gel electrophoresis, as should oligonucleotides of any length that yield anomalous results. After electrophoresis, the oligonucleotide is eluted from the gel and concentrated by reversed-phase chromatography on Sep-Pak C18 columns.