The almost full depletion (by 80 to 90%) of free SUMO following the overexpression of eYFP.PIAS4 or inhibition of the proteasome is in stark contrast to the levels of depletion during HSV-1 infection (20 to 30%) (Fig. 1B and 4B) and supports the concept that free SUMO pools are regulated during infection. Here, PIAS4 is linked to infection.