Although cmvIL-10 has not yet been quantified in patient serum, measurement of cmvIL-10 and hIL-10 may also have prognostic value in breast cancer. Elevated levels of hIL-10 (27-2134 pg/ml) have already been detected in the serum of some cancer patients and correlate with poor prognosis [81–86], suggesting that hIL-10 may contribute to immune suppression and tumor progression. In vitro, hIL-10 has been found to promote resistance to apoptosis in human breast and lung cancer cell lines [87, 88]. Importantly, recent evidence suggests that cmvIL-10 induces increased expression of hIL-10, potentially amplifying the immune suppressive environment and enabling the invasive spread of tumor cells . Our results show that cmvIL-10 increased the migration and invasive ability of MDA-MB-231 breast cancer cells and affected expression of several metastasis-related genes. Taken together, these findings suggest a new mechanism for HCMV oncomodulation, as secretion of cmvIL-10 is expected to manipulate the tumor microenvironment, enhancing the potential of a developing breast tumor to invade surrounding tissue, and ultimately establish metastatic tumors. Ultimately, it may be that a signature profile of factors like cmvIL-10, hIL-10, MMPs, and PAI-1 could have predictive or prognostic value for breast cancer. If HCMV is truly involved in promoting tumor progression, chemotherapy treatment regimens that include anti-cmvIL-10 specific antibodies or even anti-viral drugs may help improve the overall survival of cancer patients.