The present invention relates to a mutein derived from human neutrophil gelatinase-associated lipocalin (hNGAL) with affinity for the cytotoxic T lymphocyte associated antigen (CTLA-4) of KD 50 nM or less. The present invention also relates to nucleic acid molecules encoding such muteins, as well as host cells comprising such nucleic acid molecules. The present invention further relates to pharmaceutical compositions and diagnostic or analytical kits comprising said mutein.