Variations play an important role for the genetic evolution of a species. At the molecular level, genetic variations can alter the structure of a target protein through mutations in the coding region of the gene or the amount of the protein expressed by modulating gene regulation, both of which ultimately alter the function of the protein or the rate and kinetic constants in the case of an enzyme. Mutations can also modulate gene expression. Structural changes of receptors or enzymes may affect drug-receptor or drug-enzyme interaction and, consequently, drug response. 

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