Grimm et al. BMC Cancer 2010, 10:82 http://www.biomedcentral.com/1471-2407/10/82 Page 7 of 11 Figure 4 (a) Real Time PCR: Downregulation of FAS gene expression from day 10 on following an initial increase but steadily increasing of FAS-L gene expression in metastatic liver over time (day 5, 10, 15 and 20) following intraportal injection of 1 × 105 CT26.WT cells. Analysis was measured to control tissue (liver). (b) Immunohistochemistry: Increasing number (in %) of FAS-L expressing tumor cells but a decreased FAS expression from day 10 on following intraportal injection of 1 × 105 CT26.WT cells. compared to the situation in humans, reproduced prop- erly by an animal model. Thus, the main purpose of such a model should be to approximate as much charac- teristics as possible of the emergence of liver metastases. We are fully aware of the fact that our model does not represent the initial steps of dissociation, invasion and intravasation of the metastatic process [23]. Nonetheless, taking into account the prompt and reproducible initia- tion of a constant number of liver metastases following intraportal injection of CT26.WT colon cancer cells, the herein described model fulfils the demands of a sufficient and biological reliable animal model requested for the study of new therapeutical approaches. The cellular invasion with predominantly lymphocytes infiltrating the tumor is one of the main features of the host immune response. The presence of tumor specific T cells has been correlated with improved clinical out- come in different human cancers [24,25], but does not necessarily result in anti-tumor immunity since T cells can also promote the progression of tumors through dif- ferent growth factors [26]. In this regard high densities of CD3+ tumor infiltrating T cells in node negative