On the other hand, conventional 2D PHH monocultures can be infected with both viruses and are a more physiological representation of infection than transformed cell lines;25,26 however, maintaining chronic (weeks) infection in such monocultures across multiple donors is very challenging and there is a rapid decline in drug metabolism enzyme (e.g., cytochrome P450 or CYP) activities27 that do not fully mimic drug metabolism in the liver. This evidence concerns the gene PPIG and infection.