Furthermore, we hypothesize that the lessened rate of tumor regression observed for CCL-2 and IL-2 variants was also likely a result of immune system competition between clearance of foreign xenograft cells and TPV variants due to CCL-2- and IL-2-mediated immune cell recruitment and activation in combination with slower viral growth kinetics for the CCL-2 and IL-2 variants due to a lack of thymidine kinase activity. Here, CCL2 is linked to neoplasm.