A comparison of individuals infected with Hepatitis B virus (HBV), HCV, and HBV/Hepatitis D virus (HDV) co-infection revealed a variable, but significantly increased, proportion of CD4 T cells expressing perforin ex vivo compared to healthy controls; in some individuals, up to 25% of bulk CD4 T cells were perforin+ (28). This evidence concerns the gene PRF1 and coinfection.