CTLA4 (Cytotoxic T-Lymphocyte-Associated Protein 4) is a gene that encodes a protein responsible for downregulating immune responses by inhibiting T-cell activation. It competes with CD28 to bind CD80/CD86 on antigen-presenting cells, aiding in maintaining immune balance and preventing overactivation. Dysregulation of CTLA4 is associated with autoimmune diseases and impaired immune tolerance.