The term ‘gain of function’ is perhaps one of the most misunderstood in the scientific lexicon. I would like to explain what the phrase means from the perspective of a scientist who has done gain of function research for the past 40 years.

Gain of function (GoF) research gives an organism a new property or enhances an existing one. The organism can be a virus, bacterium, fungus, rodent, bird, fish or anything that can be experimentally manipulated (technically whales and elephants could be included in the definition but it would be very difficult to to GoF research on them).

Many have the impression that GoF research involves making an organism more deadly – for example, increasing the capacity of a virus to cause disease. That impression is incorrect. Certainly GoF research might lead to a more dangerous organism, but most of the time that is not the goal.

There are two broad approaches to GoF research. I’ll illustrate them with viruses but the principles could apply to any organism. In one approach, a virus is passaged in a host until a virus with different properties is obtained. An example is the adaptation of a strain of poliovirus – the type 2 Lansing strain – to replicate in mice and paralyze them. The Lansing strain does not infect mice, but an investigator passed the virus 99 times from mouse to mouse and ended up with a new strain that could now paralyze the mice. The new version of the virus had a new property – it could now infect mice. This experiment was GoF research.

Another way to do GoF research is to use recombinant DNA technology to engineer changes in the genome of the organism. In experiments done in my laboratory, we took a small piece of the genome of the mouse-adapted Lansing strain of poliovirus – coding for just eight amino acids – and spliced it into the genome of another poliovirus that is unable to infect mice. The recombinant virus from this experiment had a new property – the ability to infect mice. This experiment would also be classified as GoF.

An illustration of how GoF can be done on mice is our creation of transgenic mice susceptible to poliovirus. Mice cannot be infected with the virus because they do not produce the cell receptor for poliovirus. We introduced the human poliovirus receptor gene into the mouse germline, leading to mice that produce poliovirus receptor. After infection, these poliovirus receptor transgenic mice can be infected with poliovirus and develop paralysis. The mice have a new property – susceptibility to poliovirus infection. The mice are a product of a GoF experiment.

GoF research may have a myriad of useful outcomes. Do you want to make a different tasting beer? Modify an enzyme in the yeast used for fermentation. But in the past 30 years GoF research has received a bad name. The catalyst was a series of experiments on highly pathogenic avian H5N1 influenza viruses. These viruses rarely infect humans and do not transmit well among people. In experiments to understand what limited transmission, the virus was genetically modified and passaged among ferrets. The result was a virus that could transmit among ferrets by respiratory droplets. These GoF experiments were met with criticism, entirely unwarranted as the passaged viruses had lost their virulence for ferrets! Nevertheless since then a dark cloud has unjustifiably hung over all GoF research.

GoF research has been in the press again recently as a consequence of the COVID-19 pandemic. After the SARS-CoV pandemic of 2003, wildlife sampling efforts in China revealed many SARS-like coronaviruses in bats. To assess the potential of these viruses for infecting humans, their spike protein encoding genes were substituted into the SARS-like CoV WIV1. These recombinant viruses reproduced in human airway cells – no different from WIV1 – but at least one caused more severe disease in mice. Consequently these are GoF experiments. Some have suggested that such GoF work gave rise to SARS-CoV-2 in a lab, but this notion is impossible, as none of these viruses are close enough to be a precursor of the current pandemic virus.

The production of recombinant coronaviruses to assess pandemic potential was carried out in several laboratories, all funded by the NIH. Recently Dr. Anthony Fauci told Congress that the NIH did not fund GoF coronavirus research. The press has suggested that he lied, but the truth is that his definition of GoF research is that it only involves passaged of organisms in animals. This interpretation is not correct but being wrong does not mean you are lying.

I want readers to understand that the goals of GoF research are laudable, and only a small subset has the potential to harm humans. Consequently these experiments are highly regulated and carried out under high levels of biological containment. GoF is not a dirty word.