To calibrate the infection conditions, we infected human foreskin fibroblasts (HFFs) with a dual-color HSV-1 virus (OK41) that expresses mTurq2 (a cyan fluorescent protein [CFP]) under the immediate early cytomegalovirus (IE-CMV) promoter (expressed in similar kinetics as immediate early genes of HSV-1 [28] but independent of VP16 transactivation) and mCherry (a red fluorescent protein [RFP]) fused to the UL25 gene under the late native promoter. The gene discussed is CFP; the disease is infection.