The principle behind cancer vaccination is based on the immune system's ability to discriminate between normal and malignant cells. The immune system recognizes an altered pattern of peptide expression on the surface of the malignant cells. The altered pattern may involve the expression of qualitatively altered cell proteins, simple overexpression of normally occurring proteins, or de novo expression of proteins/glycoproteins not normally expressed in that tissue. This category of immune-recognized proteins is known as tumor associated antigens. As proteins are degraded in cells, peptides are presented on the surface of these cells as a complex with tissue type molecules (HLA molecules). T-cells may then recognize the peptide-HLA complexes, via its T-cell receptor, potentially resulting in tumor-cell killing, if sufficient priming takes place.