Pathogenic yersiniae secrete about a dozen anti-host proteins, the Yops, by a pathway which does not involve cleavage of a classical signal peptide. The Yop secretory apparatus, called Ysc, for Yop secretion, is the archetype of type III secretion systems (which serve for the secretion of virulence proteins by several animal and plant pathogens) and is related to the flagellar assembly apparatus. The Yop secretion signal is N-terminal but has not been defined to date. Apart from the Ysc machinery, secretion of at least four Yops requires cytoplasmic proteins called Syc (for specific Yop chaperone). Each Syc protein binds to its cognate Yop. Unlike most cytoplasmic chaperones, these proteins do not have an ATP-binding domain, and are presumably devoid of ATPase activity. They share a few common properties: an acidic pl, a size in the range of 15-20 kDa, and a putative amphipathic alpha-helix in the C-terminal portion. They were recently shown to have counterparts in other pathogenic bacteria, where they appear to have a similar function.