The technological challenges mentioned previously in connection with antibody and peptide microarrays are much more complex in the case of peptide-MHC microarrays, which in fact involve elements of the two previous technologies, namely presentation of peptide and antibody detection of secreted cytokine. The technology of peptide-MHC microarrays, though still in its infancy, is viewed as a simple and economical method for screening the T cell repertoire of a host [99] , and thus holds great potential. The first clinical research application of peptide-MHC microarray technology was the study of the correlates of protection regarding the effects of an experimental therapeutic cancer vaccine [104] . Ten patients with melanoma were immunized with a peptide vaccine, and their immune responses were examined with a peptide-MHC microarray, which contained seven types of peptide-MHC epitopes and probed for 26 secreted factors. This peptide-MHC array was shown to have the sensitivity to detect one peptide-MHC specific T cell in 10,000, and 10 6 CD8 þ cells were incubated with the array (so, in theory, this peptide-MHC array could detect up to 100 distinct reactive features that have reached minimum frequency of 1:10,000). Analysis of the peptide-MHC microarray response patterns demonstrated that patients who presented both IFN-c and TNF-a secretory responses against a specific epitope remained free of melanoma.