Yersinia pseudotuberculosis, a gastro-intestinal bacterium, produces three closely related T cell superantigens, YPMa, YPMb and YPMc, which have no significant sequence similarity to other proteins, let alone other bacterial superantigens. Y. pseudotuberculosisderived mitogen (YPM) has been shown to play a role in the pathogenesis of human and animal Y. pseudotuberculosis infection. The three-dimensional structure of YPMa, as determined by X-ray crystallography and nuclear magnetic resonance spectroscopy, exhibits a jelly roll fold, a structural motif not observed in other superantigens. YPMa is structurally most similar to virus capsid proteins and members of the tumour necrosis factor (TNF) superfamily. In the crystal structure, YPMa forms a trimer, another feature shared with virus capsid proteins and TNF superfamily proteins. However, in solution YPMa exists as a monomer, and any functional relevance of the trimer observed in the crystals is yet to be established. Structures of YPM bound to the T cell receptor and/or the major histocompatibility complex (MHC) are not yet available and mapping of existing mutagenesis data onto the three-dimensional structure of YPMa did not reveal potential T cell receptor/MHC binding sites. Knowledge of the structure will aid the design of functional studies aimed at further characterizing this superantigen.