At early stages of infection (3 hpi), HFF-1 infected with either HCMV US18stop, HCMV US20stop, or HCMV US18/20stop displayed a strong phosphorylation of SMAD1/5/9 already in the absence of BMP9 stimulation, potentially due to endogenously secreted BMP9 (see Fig. 3D). Here, SMAD1 is linked to infection.