a polymorphic variant of a gene can lead to the abnormal expression or to the production of an abnormal form of the protein; this abnormality may cause or be associated with disease. for example, a polymorphic variant of the enzyme cyp4a11 in which thymidine replaces cytosine at the gene's nucleotide 8590 position encodes a cyp4a11 protein that substitutes phenylalanine with serine at the protein's amino acid position 434. this variant protein has reduced enzyme activity in metabolizing arachidonic acid to the blood pressure-regulating eicosanoid, 20-hydroxyeicosatetraenoic acid. a study has shown that humans bearing this variant in one or both of their cyp4a11 genes have an increased incidence of hypertension, ischemic stroke, and coronary artery disease.