Abstract: A methods is disclosed for identifying a substance capable of disrupting an interaction between (i) a herpes simplex virus (HSV) ICP34.5 polypeptide or a homologue thereof, or a derivative thereof, and (ii) proliferating cell nuclear antigen (PCNA) or a homologue thereof, or a derivative thereof, which method comprises: (a) providing an HSV ICP34.5 polypeptide or a homologue thereof, or a derivative thereof, as a first component; (b) providing PCNA, or a homologue thereof, or a derivative thereof, as a second component; (c) contacting the two components with a substance to be tested under conditions that would permit the two components to interact in the absence of the said substance; and (d) determining whether the said substance disrupts the interaction between the first and second components.