Essential for the anterograde spread of the infection throughout the host nervous system. Together with the gE/gI heterodimer, US9 is involved in the sorting and transport of viral structural components toward axon tips. During virion morphogenesis, this protein probably accumulates in the endosomes and trans-Golgi where secondary envelopment occurs. It is probably transported to the cell surface from where it is endocytosed and directed to the trans-Golgi network (TGN), maybe through an interaction with PACS-1 sorting protein. Phosphorylated on serines within the acidic cluster, possibly by host CK2. Phosphorylation determines whether endocytosed viral US9 traffics to the trans-Golgi network or recycles to the cell membrane. Belongs to the alphaherpesvirinae envelope protein US9 family.