The lysyl oxidase family is made up of five members: lysyl oxidase (LOX) and lysyl oxidase-like 1-4 (LOXL1-LOXL4). All members share conserved C-terminal catalytic domains that provide for lysyl oxidase or lysyl oxidase-like enzyme activity; and more divergent propeptide regions. LOX family enzyme activities catalyze the final enzymatic conversion required for the formation of normal biosynthetic collagen and elastin cross-links. The importance of lysyl oxidase enzyme activity to normal bone development has long been appreciated, but regulation and roles for specific LOX isoforms in bone formation in vivo is largely unexplored. Fracture healing recapitulates aspects of endochondral bone development. The present study first investigated the expression of all LOX isoforms in fracture healing. A remarkable coincidence of LOXL2 expression with the chondrogenic phase of fracture healing was found, prompting more detailed analyses of LOXL2 expression in normal growth plates, and LOXL2 expression and function in developing ATDC5 chondrogenic cells. Data show that LOXL2 is expressed by pre-hypertrophic and hypertrophic chondrocytes in vivo, and that LOXL2 expression is regulated in vitro as a function of chondrocyte differentiation. Moreover, LOXL2 knockdown studies in vitro show that LOXL2 expression is required for ATDC5 chondrocyte cell line differentiation through regulation of SNAIL and SOX9, important transcription factors that control chondrocyte differentiation. Taken together, data provide evidence that LOXL2, like LOX, is a multifunctional protein. LOXL2 promotes chondrocyte differentiation by mechanisms that are likely to include roles as both a regulator and an effector of chondrocyte differentiation.The lysyl oxidase family is made up of five members: lysyl oxidase (LOX) 3 and lysyl oxidase-like 1-4 (LOXL1-LOXL4)(1). All five members share a conserved C-terminal catalytic domain that provides for lysyl oxidase or lysyl oxidase-like enzyme activity, and more divergent propeptide regions. LOX family enzyme activities catalyze the final enzymatic conversion required for the subsequent formation of normal lysinederived biosynthetic cross-links found in collagens and elastin (2). The importance of lysyl oxidase enzyme activity to normal bone development has long been known, and is based in part on studies in which enzyme activities are inhibited by lathyrogens in vivo (3), including -aminopropionitrile, a potent inhibitor of LOX and LOX isoform activity (4). The LOX family can be subdivided into two subgroups. Although all members have similarities in the catalytic C-terminal region, LOX and LOXL1 are more closely related to each other than they are to LOXL2-LOXL4. Moreover, the pro-peptide regions of LOX and LOXL1 have very little similarity to each other and no similarity to LOXL2-LOXL4. The pro-regions of LOXL2-LOXL4 each contain four scavenger receptor cysteine-rich (SRCR) domains whose functions are likely to depend on interactions with other proteins (1). The pro-regions of all LOX family members may have...