CTLA4 (Cytotoxic T-Lymphocyte-Associated Protein 4) is a gene that encodes a protein involved in downregulating immune responses by inhibiting T-cell activation. It competes with CD28 for binding to CD80/CD86 on antigen-presenting cells, helping to maintain immune balance and prevent overactivation. Dysregulation of CTLA4 is linked to autoimmune conditions and impaired immune tolerance.