The Dorset outbreak strain was closely related to the common STEC O157:H7 clone and shared several characteristics, most notably the presence of phage-encoded stx2a. Stx2a is associated with more severe symptoms, including the development of HUS, and it is probably the key virulence factor causing the high proportion of HUS cases in this outbreak (10). The gene discussed is STX2; the disease is hemolytic-uremic syndrome.