CHO cells, short for Chinese hamster ovary cells, are the workhorses of the pharmaceutical industry. Roughly 70% of all recombinant therapeutic proteins approved for human use are manufactured in these cells, including blockbuster drugs you’ve almost certainly heard of. They’re also widely used in basic genetics research, vaccine development, and toxicology testing.
Origins as a Genetics Tool
CHO cells trace back to the late 1950s, when a culture in the laboratory of Theodore Puck at the Eleanor Roosevelt Institute for Cancer Research in Denver, Colorado spontaneously became immortal, meaning the cells could divide indefinitely rather than dying off after a set number of generations. Scientists quickly noticed that CHO cells had unusually large chromosomes, visible under a standard light microscope, which made them ideal for studying mammalian genetics.
Researchers could also create metabolic mutants of these cells relatively easily. Some of those mutants corresponded to visible chromosomal rearrangements, which allowed scientists to map genes to specific locations on chromosomes and study what those genes actually did. This made CHO cells a foundational tool for understanding how mammalian genes work, well before the cells became central to drug manufacturing.
The Dominant Platform for Drug Manufacturing
The biggest use of CHO cells today is producing therapeutic proteins. Nearly 80% of approved human therapeutic antibodies are made in CHO cells. In 2021 and 2022, virtually every new recombinant antibody approved by the U.S. or European Union was manufactured in a CHO cell line, with only a single exception.
The range of products goes far beyond antibodies. CHO cells produce:
- Monoclonal antibodies: adalimumab (Humira), rituximab, trastuzumab (Herceptin), bevacizumab (Avastin), ipilimumab, and dozens more
- Fc-fusion proteins: etanercept (Enbrel), abatacept, aflibercept
- Cytokines and growth factors: erythropoietin (epoetin alfa), darbepoetin alfa, interferon beta-1a
- Enzymes: alteplase (a clot-dissolving drug), imiglucerase (for Gaucher disease), alglucosidase alfa (for Pompe disease)
- Hormones: follitropin alfa and beta (fertility treatments), thyrotropin alfa
- Clotting factors: Factor VIII and Factor IX for hemophilia
The monoclonal antibody market alone was projected to exceed $300 billion by 2025, and CHO cells are the engine behind the vast majority of that production.
Why CHO Cells Beat the Competition
Several biological quirks give CHO cells a manufacturing edge. First, they can be adapted to grow in suspension in chemically defined, serum-free media. This matters because serum (derived from animal blood) introduces batch-to-batch variability and contamination risk. When serum concentration drops below about 5%, CHO cells naturally shift from sticking to surfaces to floating freely in liquid, which is exactly what you want for large-scale production in stirred bioreactors.
Second, CHO cells add sugar molecules to proteins in patterns that closely resemble human patterns. This “glycosylation” is critical because it affects how long a drug stays active in the body and whether the immune system tolerates it. Bacteria like E. coli can produce proteins cheaply but cannot perform this modification, which is why complex therapeutic proteins require mammalian cells.
Third, CHO cells have a restricted susceptibility to human viruses. Because they’re rodent cells, most human pathogens simply can’t replicate in them, which provides a built-in safety margin for the final drug product. Regulatory agencies still require adventitious agent testing of every batch, but systematic screening has confirmed that CHO-derived products carry very high safety margins.
How Scientists Engineer CHO Cells
Getting CHO cells to produce large quantities of a specific protein requires inserting the gene for that protein and then selecting for cells that make a lot of it. Two main selection systems dominate the industry.
The first uses a gene called DHFR. Scientists work with CHO cells that lack the ability to make their own nucleosides (building blocks for DNA). They introduce the protein gene alongside a working copy of DHFR, then grow the cells in media without nucleosides. Only cells that took up the new genes survive. To push production higher, they expose the cells to increasing doses of a drug called methotrexate, which blocks DHFR. The cells respond by duplicating the region of DNA containing both the DHFR gene and the product gene, resulting in more copies and higher output.
The second system uses a gene called GS, which cells need to make the amino acid glutamine. Cells that successfully incorporate the gene of interest along with GS can survive in the presence of an inhibitor that blocks normal glutamine production. Raising the inhibitor concentration selects for cells with amplified gene copies, again boosting protein yield.
Industrial-Scale Production
Commercial manufacturing typically uses fed-batch processes in large stainless-steel or single-use bioreactors. Production-scale vessels commonly reach 5,000 liters or more, though scaling up introduces real engineering challenges. Larger bioreactors have lower oxygen transfer, slower mixing, and reduced carbon dioxide removal compared to the bench-scale vessels used during development. pH and dissolved oxygen can vary across different zones of a 5,000-liter tank, which means the cells in one part of the reactor may experience different conditions than cells elsewhere.
To maximize yield, manufacturers carefully control temperature, nutrient feeding schedules, and media composition. Lowering temperature during production, for instance, slows cell growth but can increase the amount of protein each cell produces while also reducing protein degradation. Adding trace metals like zinc has been shown to double the activity of certain enzymes produced in CHO cultures. A typical fed-batch run lasts about 14 days.
Gene Editing Is Reshaping CHO Engineering
Since about 2012, CRISPR gene editing has transformed how scientists optimize CHO cells. From 2020 through 2024, every published study that knocked out genes in CHO cells used CRISPR. The technology allows researchers to precisely disable genes that limit protein production or compromise product quality.
The results have been striking. Knocking out two genes involved in epigenetic silencing improved antibody production 3.9-fold in one study. Disabling three genes related to cell stress and programmed cell death boosted antibody output 3.5-fold. Targeting a component of the cell’s energy machinery increased fusion protein production 3.1-fold. Overall, engineering strategies focused on epigenetics, combining multiple gene targets, and controlling cell death have yielded the highest improvements, with median gains of roughly 2.2 to 2.7 times above baseline.
Beyond simple knockouts, CRISPR has enabled genome-wide screening to discover entirely new targets for engineering. Researchers can systematically activate silenced genes across the genome and identify which ones boost production, uncovering targets that no one would have guessed from existing knowledge alone.