For instance, the presence of HSV-1 DNA or IgG seropositivity with ApoE4 gene has been shown to pose a greater risk factor in AD development than either one by itself (Lin et al., 1995, 1996; Itzhaki et al., 1997; Steel and Eslick, 2015; Lopatko Lindman et al., 2019). The gene discussed is APOE; the disease is Alzheimer disease.