Disclosed are protein ligands comprising an immunoglobulin heavy chain variable (VH) domain and an immunoglobulin light chain variable (VL) domain, wherein the proteins bind a complex comprising an MHC and a peptide, do not substantially bind the MHC in the absence of the bound peptide, and do not substantially bind the peptide in the absence of the MHC, and the peptide is a peptide fragment of gp100, MUC1, TAX, or hTERT. Also disclosed are methods of using and identifying such ligands.