evidence: The genome sequence of a virus is the sequence or order of bases or letters that make up a virus’s genetic material, or its genome. If you were to write down thegenome sequence of a particular coronavirus, it would be a series of about 30,000 letters. The key is to get them in just the right order that you find them in a particular virus. [...] Genome sequencing is how scientists found this new human coronavirus soon after it popped up in people. At first, what they knew was that people in China were suddenly getting sick with respiratory symptoms. So scientists sequenced the genome of a viral sample from a person who worked at a market where they thought it might have come from. By comparing the RNA sequence to other viral genome sequences they had from earlier studies, they could tell right away that it was a coronavirus they hadn’t seen in people before. [...] They knew the spike would be important for the virus to infect our cells. They also knew it could be a useful target for vaccines or other treatments. They knew all of this soon after discovering the coronavirus because they could sequence its genome and compare the genome to other viral genomes.