These represent a range of standardized assays designed to test chemicals for interaction with DNA, that is, genotoxicity or mutagenicity. These comprise a number of different tests both in vitro and in vivo. The most usual study conducted early in the development of a new drug is the Ames assay that utilizes mutants of Salmonella typhimurium. Various mammalian cell culture systems are also employed such as Chinese hamster ovary (CHO) cells. Clastogenic activity is assessed by exposing cells to chemicals and examining cells microscopically for chromosome damage. An in vivo study is also often performed by examination of the bone marrow in treated rodents.