In biology, and especially in genetics, a mutant is an organism or a new genetic character arising or resulting from an instance of mutation, which is generally an alteration of the DNA sequence of the genome or chromosome of an organism. It is a characteristic that would not be observed naturally in a specimen. The term mutant is also applied to a virus with an alteration in its nucleotide sequence whose genome is RNA, rather than DNA. In multicellular eukaryotes, a DNA sequence may be altered in an individual somatic cell that then gives rise to a mutant somatic cell lineage as happens in cancer progression. Also in eukaryotes, alteration of a mitochondrial or plastid DNA sequence may give rise to a mutant lineage that is inherited separately from mutant genotypes in the nuclear genome. The natural occurrence of genetic mutations is integral to the process of evolution. The study of mutants is an integral part of biology; by understanding the effect that a mutation in a gene has, it is possible to establish the normal function of that gene.