Source:http://linkedlifedata.com/resource/umls/id/C1868500
NCI: Variation in the manner in which a trait is manifested. When there is variable expressivity, the trait may vary in clinical expression from mild to severe. For example, the condition neurofibromatosis type 1 may be mild, presenting with cafe-au-lait spots only, or may be severe, presenting with neurofibromas and brain tumors.