Mutagenesis may occur endogenously (e.g. spontaneous hydrolysis), through normal cellular processes that can generate reactive oxygen species and DNA adducts, or through error in DNA replication and repair. Mutagenesis may also occur as a result of the presence of environmental mutagens that induce changes to an organism's DNA. The mechanism by which mutation occurs varies according to the mutagen, or the causative agent, involved. Most mutagens act either directly, or indirectly via mutagenic metabolites, on an organism's DNA, producing lesions. Some mutagens, however, may affect the replication or chromosomal partition mechanism, and other cellular processes.