In an effort to identify tumor-specific antigens recognized by CD4⁺ T cells, an approach was developed that allows the screening of an invariant chain–complementary DNA fusion library in a genetically engineered cell line expressing the essential components of the major histocompatibility complex (MHC) class II processing and presentation pathway. This led to the identification of a mutated form of human CDC27, which gave rise to an HLA-DR4–restricted melanoma antigen. A mutated form of triosephosphate isomerase, isolated by a biochemical method, was also identified as an HLA-DR1–restricted antigen. Thus, this approach may be generally applicable to the identification of antigens recognized by CD4⁺ T cells, which could aid the development of strategies for the treatment of patients with cancer, autoimmune diseases, or infectious diseases.