 TSST-O does not cause TSS, and is non-mitogenic, and differs in sequence from TSST-1 in 14 nucleotides, which corresponds to 9 amino acids.Two of these are cleaved off as part of the signal sequence, and are therefore not important in the difference in function observed.From the studies observing the differences in these two proteins, it was discovered that residue 135 is critical in both lethality and mitogenicity, while mutations in residues 132 and 136 caused the protein to lose its ability to cause TSS, however there were still signs of superantigenicity.