Our scientific activity belongs to tumor immunology: in the last fifteen years, we have contributed to the molecular definition of tumor antigens recognized by cytolytic T lymphocytes, both in human and mice, and to the validation of cancer immunotherapy in appropriate preclinical models. Here we describe two findings that we recently made in this field. The first is the discovery of a new mechanism of production of antigenic peptides, based on the splicing of peptides by the proteasome. The second is the discovery of an important mechanism of tumor resistance to immune rejection. The major interest of this mechanism is that it can be fought with pharmacological agents, which could be used to boost the efficacy of immunotherapy in cancer patients.