Whereas the US28 and IE1 gene products are expressed in infected cells, the UL111A gene encodes cmvIL-10, a viral cytokine that is secreted from infected cells. Although cmvIL-10 has only 27% sequence identity to human interleukin-10 (hIL-10) , the viral cytokine binds to the cellular IL-10 receptor with greater affinity than hIL-10 itself . Extensive immunosuppressive properties of cmvIL-10 have been documented, including inhibition of inflammatory cytokine synthesis, downregulation of class I and II MHC, and inhibition of dendritic cell maturation [41–48]. Engagement of the IL-10 receptor by cmvIL-10 leads to activation of Stat3 [49–53], which is commonly constitutively activated in breast cancer cells , associated with poor prognosis in ovarian cancer, and considered a key factor in metastasis formation . CmvIL-10 was found to activate Stat3 and play a pivotal role in the progression of malignant glioma by enhancing the invasiveness and migration of glioma cancer stem cells . Because cmvIL-10 is secreted from the infected cell, it has the potential to act on any cell type, infected or not, that expresses the IL-10 receptor.