An oligo is a short chain of linked molecular units, most commonly referring to an oligonucleotide: a short, synthetic strand of DNA or RNA typically 15 to 25 building blocks (nucleotides) long. The prefix “oligo” comes from the Greek word “oligos,” meaning “few” or “scanty,” so an oligonucleotide is literally “a few nucleotides.” You’ll encounter this term in genetics labs, medical research, and increasingly in pharmacy, where oligos are used as both research tools and prescription drugs.
While “oligo” can technically refer to any short molecular chain (oligosaccharides are short sugar chains, oligopeptides are short protein chains), in practice, when scientists and doctors say “oligo,” they almost always mean oligonucleotide. That’s the usage this article focuses on.
How Oligos Are Built
Natural DNA and RNA in your cells can be billions of nucleotides long. Oligos, by contrast, are deliberately short, usually between 15 and 100 nucleotides. A typical 20-nucleotide oligo weighs about 7 kilodaltons and carries 19 negative electrical charges, making it a tiny, highly charged molecule compared to the massive chromosomes in your cells.
Synthetic oligos are manufactured using a chemical process called phosphoramidite chemistry, introduced in 1981 and still the standard method today. The process works on a solid support (think of it like building a chain one bead at a time on a fixed surface) through a repeating four-step cycle. Each cycle adds one nucleotide to the growing chain. Modern automated machines can complete an entire oligo in hours, and companies ship simple custom oligos the same day they’re ordered.
Researchers order oligos the way you might order business cards: specifying the exact sequence of letters (A, T, G, C for DNA), the quantity, and the level of purification. Common order sizes range from 25 nanomoles for small experiments up to 10 micromoles for large-scale work. The final yield depends on the length and purification method chosen.
What Oligos Do in the Lab
The most common use of oligos is as primers and probes, two workhorses of molecular biology. A primer is a short oligo that tells a DNA-copying machine exactly where to start reading. This is the foundation of PCR (polymerase chain reaction), the technique behind COVID tests, forensic DNA analysis, and genetic research. Without primers, there’s no way to selectively copy a specific stretch of DNA out of an entire genome.
A probe works differently. It’s an oligo labeled with a fluorescent or radioactive tag that binds to a matching DNA or RNA sequence, lighting up its location. Probes can identify specific bacteria by targeting signature genes unique to each species. They’re used in diagnostic tests to confirm infections, detect genetic mutations, and identify organisms in environmental samples. Their small size gives them an advantage over larger molecules: they penetrate tissue more easily and bind their targets more reliably.
Oligos as Medicine
Beyond the lab bench, oligos have become a growing class of drugs. As of April 2025, the FDA has approved 22 oligonucleotide therapies. These drugs work by interacting directly with RNA inside your cells, essentially intercepting genetic messages before they can be carried out.
There are three main strategies. The first uses antisense oligonucleotides (ASOs), single-stranded oligos that bind to a specific RNA message and either trigger its destruction or physically block it from being read. When an ASO binds to its target RNA, it can recruit a natural cellular enzyme that chops up the RNA, preventing the faulty protein from ever being made. Alternatively, the ASO can simply sit on the RNA like a roadblock, redirecting how the cell processes the message. This “steric blocking” approach is particularly useful for diseases caused by errors in how genes are spliced together.
The second strategy uses small interfering RNAs (siRNAs), which are double-stranded oligos that hijack the cell’s own gene-silencing machinery. Once inside a cell, the two strands separate, and one strand guides a protein complex to find and destroy the matching RNA message. This approach harnesses a natural defense system that cells already use to regulate gene activity.
The third strategy, still less common in approved drugs, uses aptamers: oligos folded into specific 3D shapes that can grab onto proteins the way antibodies do, blocking their activity directly.
Why Raw Oligos Don’t Survive in the Body
Unmodified DNA and RNA strands are fragile. Your body is full of enzymes (nucleases) whose job is to chew up stray genetic material. An unmodified oligo injected into the bloodstream would be destroyed in minutes. This was the central challenge in turning oligos into drugs.
The solution came through chemical modifications that make oligos tougher without changing their ability to recognize targets. The most important modification swaps one of the oxygen atoms in the oligo’s backbone for a sulfur atom, creating what’s called a phosphorothioate linkage. This single-atom change dramatically increases stability and helps the oligo bind to blood proteins like albumin, which acts as a carrier and extends the oligo’s life in circulation. Research has shown that as few as three of these modified linkages at each end of the oligo are enough to provide full protection against the enzymes that would otherwise degrade it from the ends inward.
Other modifications target the sugar portion of each nucleotide. Adding a small chemical group to a specific position on the sugar ring improves how tightly the oligo grips its target RNA and further resists enzymatic breakdown. The approved drugs on the market today rely on just a handful of these modifications, most of which were developed over 50 years ago. A typical therapeutic oligo combines backbone modifications in certain regions with sugar modifications in others, creating a hybrid design optimized for both target recognition and durability.
Diseases Treated With Oligo Drugs
The 22 approved oligo therapies target a range of conditions that were previously difficult or impossible to treat, many of them rare genetic diseases. These conditions share a common thread: they’re caused by a specific, identifiable genetic error that produces a harmful protein or fails to produce a necessary one. Because oligos can be designed to match virtually any RNA sequence, they offer a way to address the root genetic cause rather than just managing symptoms.
One landmark example is a siRNA-based drug delivered using tiny fat particles (lipid nanoparticles) that carry the oligo into liver cells, where it silences a gene responsible for a condition that causes dangerous protein buildup in organs. This drug demonstrated that the delivery problem, getting oligos into the right cells, could be solved at a practical level. Other approved oligos treat conditions affecting the nervous system, muscles, eyes, and liver, with the list continuing to expand as new targets are validated.
Other Types of Oligos
While oligonucleotides dominate the conversation, the “oligo” prefix appears in other biological contexts. Oligosaccharides are short chains of 2 to 10 sugar molecules linked together. Common examples include sucrose (table sugar), lactose (milk sugar), and maltose. Some oligosaccharides are digestible and provide energy, while others pass through your stomach undigested and serve as food for beneficial gut bacteria. These “functional” oligosaccharides, including fructo-oligosaccharides found in many prebiotic supplements, promote healthy intestinal flora.
Oligopeptides are short chains of amino acids, and oligomers is the general chemistry term for any short repeating chain. But if you hear “oligo” in a biology, genetics, or medical context, it almost certainly refers to an oligonucleotide.