Lumican (LUM), a member of the small leucine-rich proteoglycan (SLRP) family, is one of the major extracellular components in interstitial collagenous matrices of the corneal stroma, aorta, skin, skeletal muscle, lung, kidney, bone, cartilage, and intervertebral discs. SLRPs constitute an important fraction of noncollagenous extracellular matrix proteins and have important effects on cell behavior. Lumican regulates collagenous matrix assembly as a keratan sulfate proteoglycan in the cornea and may exist as a glycoprotein in the connective tissues of other organs. Posttranslational modifications lumican goes through can increase its apparent molecular weight to 50-90 kDa. Studies show that lumican participates in the maintenance of tissue homeostasis and modulates cellular functions including cell proliferation, migration, and differentiation.