Collectively, we demonstrate for the first time that by strategic combination of HLA class II-transgenic mice, immunization with recombinant Ad5 and combinatorial antigen-specific peptide library screening, CD4+ T cell epitopes from melanoma differentiation antigens can readily be defined, pointing to a broad applicability of this approach to target antigens of other tumor entities and to different HLA class II molecules even without prior characterization of their peptide binding motives. The gene discussed is CD4; the disease is melanoma.