M protein is the major virulence factor in S. pyogenes. It extends from the cell wall as a hair-like projection and disrupts phagocytosis. Organized as a rod-like coiled structure separated into functional domains, M protein can accumlate multiple sequence changes while maintaining function. There are two main structural classes, I and II. Pili are also covered with lipoteichoic acid, which facilitates attachment to epithelial cells.