Genes are units of genetic material present on the chromosomes. They code for molecules of RNA called mRNA that code for a particular protein. This protein expresses itself either in the form of a phenotypic character or as a metabolite that plays a role in some metabolic pathway. Mutations in the genetic material of an organism often result in alteration of the expression of a gene. This alteration could silence the gene, reduce the expression of the gene, or enhance expression of a gene. Such alterations are often observed in the form of diseases.