The concept of tumor vaccines is not new. However, advances in gene transfer technology, tumor immunology, molecular biology, and methods of monitoring antitumor response, have allowed for novel, more specific vaccine approaches. For example, first-generation tumor vaccines were composed of whole inactivated cancer cells, or tumor lysates (Tuly) given together with immune adjuvants like bacillus Calmette-Guerin (BCG). Current strategies include tumor cells modified with genes encoding molecules necessary to stimulate a cytotoxic T cell response, such as cytokine genes, foreign HLA genes, tumor-associated antigen (TAA) genes, and even costimulatory molecules. Activation of cellular immunity requires at least three synergistic signals including presentation of specific tumor antigens, costimulatory signals (B7 molecules), and propagation of the immune response via cytokine release. In general, tumor cells often fail to demonstrate any of these immunostimulatory properties. Dendritic cell-based vaccines are gaining popularity as these cells can properly present TAA to the immune system, thus circumventing the poor antigen-presenting qualities of tumor cells. Dendritic cells can be "loaded" with TAA or other molecules either by their natural endocytotic capabilities, or by genetic modification.