> We did not find the 19-nucleotide sequence CTCCTCGGCGGGCACGTAG in any eukaryotic or viral genomes except SARS-CoV-2 with 100% coverage and identity in the BLAST database (Supplementary Tables 1–3).

Huh? I just run BLAST (blastn, specifically) with that nucleotide sequence and found several eukaryotic genomes containing it in non MutS-like contexts, aligning sense and anti-sense strands, with 100% coverage and 100% identity. Besides, there is no “BLAST” database (I used the non-redundant -nr- one, for instance; BLAST is just the name of the tool). One possible reason they didn’t find the sequences is that BLAST, by default, only returns the first 100 hits, which in their case might have been prokaryotic ones. I didn’t check viral genomes but might later.

Disclaimer: I know and have used BLAST since my undergrad years, about a decade and a half ago.