CTLA4 and neoplasm: However, soluble cytotoxic T-lymphocyte antigen 4 (sCTLA-4), one of the isoforms of CTLA-4, plays an important role in down-regulating the negative signal of CTLA-4 in T-cell responses and studies have suggested the favourable prognostic value of serum sCTLA-4 level in tumor patients10.