An Oligonucleotide is a short nucleic acid polymer, typically with twenty or fewer bases. Although they can be formed by bond cleavage of longer segments, they are now more commonly synthesized by polymerizing individual nucleotide precursors. Automated synthesizers allow the synthesis of oligonucleotides up to 160 to 200 bases. The length of the oligonucleotide is usually denoted by "mer". Because oligonucleotides readily bind to their respective complementary nucleotide, they are often used as probes for detecting DNA or RNA.