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 accumulates in the trans-Golgi where secondary envelopment occurs. 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.