A comparison of spike glycoprotein genes from different coronaviruses showed that SARS-related coronavirus is genetically distinct from other known coronaviruses. The spike glycoprotein genes of SARS coronavirus strains are highly similar to each other, indicating a unique evolutionary path. In contrast, the genes of human, avian, feline, bovine, canine, and other coronaviruses differ significantly from those of SARS coronavirus. This suggests that SARS-related coronavirus is a novel virus that has not been previously encountered by humans.