Specifically, he noticed that a section of that protein, called M2e, remains stable even as the other viral surface proteins mutate. (scientificamerican.com)
The genetic material of the virus is depicted in the doily's center, and viral surface proteins appear as protuberances around the edge. (discovermagazine.com)
Mutations to these genes also represent a primary source for immune evasion (because they are the dominant proteins exposed on the viral surface) and why we need a new flu shot every year (i.e. because the dominant circulating strains are different from what our immune system has seen before). (religion.blogs.cnn.com)