Fluorescent proteins have the properties of being very bright with high quantum yield and are available in many colors. Tumor-host models consist of transgenic mice expressing green fluorescent protein (GFP) in essentially all cells and tissues or expressing GFP selectively in specific tissues such as blood vessels. Particularly useful are the corresponding nude mice transgenic for GFP expression, as they can accept human tumors. When tumor cells expressing red fluorescent protein are implanted in mice expressing GFP, various types of tumor-host interactions can be observed, including those involving host blood vessels, lymphocytes, tumor-associated fibroblasts, macrophages, dendritic cells and others. The 'color-coded' tumor-host models enable imaging and therefore a deeper understanding of the host cells involved and their function in tumor progression. Approximately 4-8 weeks are needed for these procedures.