This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. This project investigates the structure and expression of an atypical T cell receptor (TCR) chain named TCR[unreadable]. TCR[unreadable] was discovered in marsupials, a lineage of mammals separarted from eutherians (placental mammals) by 135 to 160 million years. Like all jawed vertebrates, marsupials express [unreadable][unreadable] and [unreadable][unreadable] T cells, but also would appear to have an additional T cell lineage that are TCR[unreadable]+. TCR[unreadable] is present in all living marsupial species and has now been shown to also be present in the third living mammalian lineage, the monotremes (such as the duckbill platypus), consistent with it being present in the last common ancestor of all extant mammals, including the eutherians (placental mammals). In this project the phenotype of TCR[unreadable]+ T cells will be investigated primarily using the model marsupial species, Monodelphis domestica, the gray, short-tailed opossum. In addition, the structure of the receptor complex containing TCR[unreadable] will be determined. Key to these experiments is ongoing reagent development to characterize opossum T cells.