In other cases, individual genes have been associated with independent but related disorders that may share fundamental biological pathways in disease etiology, such as HLA-DQB1, CTLA4, and PTPN22 as in the case of autoimmune disorders. This gene overlap emphasizes the fundamental, often step-wise biochemical role each gene plays in shared disease etiology [24] [25] [26] [27] . That is, HLA-DQB1 in antigen presentation, CTLA4 in regulation of the expansion of T cell subsets, and PTPN22 in T cell receptor signaling, all contributing to immunological aberrations and progression to clinical disease, as in rheumatoid arthritis, systemic lupus erythematosus, and type 1 diabetes. In other cases, the same gene has been associated with quite different clinical phenotypes, suggesting sharing of complex biological mechanisms at a more underlying level. For example, the gene CFTR, widely recognized as the cause of cystic fibrosis, has been consistently associated with pancreatitis, may be implicated in chronic rhinitis [28] , and may play a protective role in gastrointestinal disorders [29] .