The diagnosis and treatment of acute cellular rejection remains a clinical management priority that is currently based on transbronchial biopsy. Gene expression profiling is a technology based on molecular biology that measures changes in the RNA levels of different genes expressed by circulating mononuclear cells in the peripheral blood. A key goal of the project is to use gene expression profiling to differentiate between the absence and presence of lung allograft acute cellular rejection, with the aim of using this novel information to develop a non-invasive diagnostic testing alternative.