To further recapitulate the tumor microenvironment, we examined whether cmvIL-10 could also promote invasion through matrigel, a gelatinous protein mixture derived from mouse sarcoma cells widely used to simulate the ECM in vitro . MDA cells were place atop a matrigel-coated transwell system with EGF placed in the lower chambers. Purified recombinant cmvIL-10 or hIL-10 was added to both chambers. After incubation for 22 h, invasion was assessed by counting cells in the lower chamber, which should contain only the cells that were able to degrade the matrigel coating to access the porous filter. As shown in Fig. 1b, cmvIL-10 was found to be a strong enhancer of cell invasion. Surprisingly, cmvIL-10 was able to increase invasion of MDA breast cancer cells to a significantly greater extent than hIL-10, suggesting that the viral cytokine may trigger signaling events that are distinct from the cellular cytokine. Since activation of the transcription factor Stat3 by cmvIL-10 is well-documented [39, 49–51, 53, 60, 61], we next examined the need for Stat3 in cmvIL-10-enhanced invasion. Treatment with a Stat3 inhibitor reduced the cmvIL-10-induced increase in invasion through matrigel toward EGF seen when either recombinant purified protein or cytokine produced during virus infection were present (Fig. 1c). Taken together, these results demonstrate the novel finding that not only does cmvIL-10 produced during virus infection stimulate enhanced migration and invasion of breast cancer cells, but it does so more effectively than hIL-10.