CD4 and inflammatory bowel disease: GWAS variants associated with common immune-mediated diseases, such as inflammatory bowel disease (IBD), type 1 diabetes (T1D), and rheumatoid arthritis (RA), are enriched in active chromatin marks that tag enhancers and promoters in the CD4+ T cells, especially in regulatory T cells (Treg cells).4, 5, 6 Treg cells are an infrequent yet functionally significant subset of CD4+ T cells; they comprise 2%–10% of CD4+ T cells and play an essential homeostatic role in the immune system by suppressing the proliferation and effector functions of conventional T cells.