The FCS’s two arginines use CGG-CGG - a codon pair rare in these viruses and the only doubled CGG in the genome - and the insert conveniently carries a FauI restriction site usable for cloning/screening (RFLP), a pattern the authors argue is more readily explained by human codon choice and restriction-site design than by natural evolution.
Reasoning
In these coronaviruses arginine is only ~5% encoded by CGG, so an adjacent CGG-CGG pair - the sole doubled CGG in the entire genome - inside a brand-new insert is unusual for natural sequence, whereas researchers cloning from synthetic DNA commonly use whatever codons are convenient irrespective of host preference. In addition, the insert introduces a FauI restriction site; because the new furin site is prone to deletion in vitro, a nearby restriction site is just what would let a researcher use restriction-fragment-length-polymorphism methods to clone or screen for the insert. The authors argue the coincidence of an atypical codon choice and a functionally useful restriction site at the engineered locus fits a laboratory insertion better than spontaneous natural mutation.
Validity verdict — approved (checked)
Reconstruction. Premises: (i) arginine is only ~5% CGG-encoded in these CoVs, and the insert’s CGG-CGG is the sole doubled CGG in the genome; (ii) the insert introduces a FauI restriction site; (iii) the new furin site is deletion-prone in vitro, so a nearby restriction site is useful for RFLP-based cloning/screening. Load-bearing step: from “atypical codon usage + a functionally convenient restriction site at the insert” to “the pattern is more expected under deliberate lab insertion than natural evolution.”
Evaluation. This is a comparative-likelihood claim already pitched at the modest “more readily explained / more expected” level (not a proof), so there is no over-reach to correct down. Conditional on the premises the direction holds: designed sequences do use convenient (host-non-preferred) codons and researchers do exploit restriction sites, so P(features | lab) ≥ P(features | nature). Probed undercutting defeaters: (a) a doubled rare codon can arise naturally — 5% usage makes it unusual but far from impossible; (b) FauI (and some) restriction sites occur by chance every few hundred bp, so a site “near” the insert is close to guaranteed and thus nearly non-discriminating. These defeaters attack magnitude, and (b) drains most of the evidential weight from the FauI half, but neither reverses the direction; the codon-usage half still tilts toward design. So the argument is valid-but-weak, which is approved (its strength — likely small — is priced at step 8), not corrected, since the statement does not overclaim. Traced here → checked.