The origin of the PCS is interesting, given that recombination occurs only moderately in human coronaviruses (Pollett et al.
2021). The possibility has been raised that the PCS was artificially inserted into the spike protein in a gain of function (GOF) experiment, prior to entry of the virus into the human population (Segreto and Deigin
2020). Indeed, SARS-CoV-1 spike protein has had a PCS inserted in a GOF experiment, albeit in pseudotyped lentiviruses which are safer to use (Follis et al.
2006). With SARS-CoV-2, the GOF scenario is difficult to examine from sequence analysis alone given the short length of the PCS, which means that it effectively 'blends' into the much longer spike protein backbone sequence. Whether introduced by human agency or natural selection, this sequence conformity would act to enhance functional compatibility with the rest of the spike protein. ‘No-see-um' approaches leave no trace of artificial ligation as the restriction sites do not remain in the final sequence after ligation; this approach has been used previously for altering the SARS-CoV-1 genome (Baric and Sims
2005). The rationale for using 'no-see-um' approaches for coronavirus genome manipulation are unclear. If the PCS was inserted for malign purposes, this blending effect would have a deceptive role in obscuring its engineered origin, representing a form of deceptive cue mimicry.
In addition to direct genetic manipulation of the virus, another potential GOF scenario is that of serial passage of the virus ancestor through humanized mice (Sirotkin and Sirotkin
2020). One purpose of serial passage experiments is to observe how pathogens might adapt to a new host after an initial zoonotic host jump (Ebert
1998), while the use of humanized mice allows mutations that might lead enhanced infectivity in humans to be identified. For example, in serial passage experiments in humanized mice, simian immunodeficiency virus (SIV) was evolved into HIV (Schmitt et al.
2018). Escape of such an enhanced pathogen has the potential to cause an epidemic, although HIV is less contagious than a respiratory virus.