ACVR1 and disease arising from reactivation of latent virus: Therefore, downregulation of ALK1 and ALK2 by US18 /US20 may impair the antiviral effects of BMP9, but allow signaling by other BMP/Activin/TGF-β ligands (Yu et al, 2005), which may preserve the ability of HCMV to establish latent infections.