However, cell lines obtained from such collections, can still be contaminated by viruses because viruses, which do not lead to a cytopathic effect, are not detected by the tests commonly used, on one side (ATCC), or only tests for detecting human pathogenviruses have been performed on the other side (DSMZ). If a very important cell line of a research laboratory has to be more rigorously screened, commercial screening services are needed and their use is recommended.