 Domain A, the larger of the two domains, contains residues 1-17 and 90-194 in TSST-1 and consists of a long alpha (α) helix with residues 125-140 surrounded by a 5-strand beta (β) sheet.Domain B is unique because it contains residues 18-89 in TSST-1 and consists of a (β) barrel made up of 5 β-strands.Crystallography methods show that the internal β-barrel of domain B contains several hydrophobic amino acids and hydrophilic residues on the surface of the domain, which allows TSST-1 to cross mucous surfaces of epithelial cells.