A 6.6-kDa protein component of VERY-LOW-DENSITY lipoproteins; INTERMEDIATE-DENSITY lipoproteins; and HIGH-DENSITY lipoproteins. Apo C-I displaces APO E from lipoproteins, modulate their binding to receptors (receptors, ldl), and thereby decrease their clearance from plasma. Elevated Apo C-I levels are associated with HYPERLIPOPROTEINEMIA and atherosclerosis.