The ETT2 gene cluster, encoding a second type III secretion system from Escherichia coli, is present in the majority of strains but has undergone widespread mutational attrition.
Ren CP, Chaudhuri RR, Fivian A, Bailey CM, Antonio M, Barnes WM, Pallen MJ. J Bacteriol. 2004 Jun; 186(11):3547-60.