When a producer cell line of murine origin is used, the consensus opinion among regulators is that all known murine viruses should be tested for. If a cell of human origin is involved in production, then there should be tests for human viruses, such as HIV, HTLV, EBV, CMV, HHV6 and HHV7. Human-mouse heterohybridoma cell lines have to be tested for both human and murine viruses (Robertson, 1996) .