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 C.P., Chaudhuri R.R., Fivian A., Bailey C.M., Antonio M., Barnes W.M., & Pallen M.J. (2004). “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. J Bacteriol. 2004 Jun;186(11):3547-60. (Abstract)