Abstract Lysyl oxidases (EC 1.4.3.13, protein-lysine 6-oxidases) are extracellular copper enzymes that initiate the cross-linking of collagens and elastin by catalyzing oxidative deamination of the ε-amino group in certain lysine and hydroxylysine residues. The cross-links formed are responsible for the tensile strength of collagen fibers and the unique elastic properties of elastin. Three human lysyl oxidase isoenzymes, lysyl oxidase (LOX), lysyl oxidase-like protein (LOXL), and lysyl oxidase-like 2 protein (LOXL2), have been identified and characterized so far. Two additional human lysyl oxidase isoenzymes, lysyl oxidase-like 3 (LOXL3) and lysyl oxidase-like 4 (LOXL4), proteins were identified, cloned, and partially characterized in this study. Both polypeptides showed a high degree of overall similarity to each other and to the LOXL2 polypeptide, whereas the two polypeptides showed a significant similarity to LOX and LOXL only in the C-terminal region, which contains all amino acid residues thought to be needed for the catalytic activity of the LOX enzyme. The LOXL3 gene is expressed in several tissues, the highest expression levels being in the placenta, heart, ovary, testis, small intestine, and spleen. The LOXL4 gene is likewise expressed in most human tissues studied, the highest levels being seen in the skeletal muscle, testis, and pancreas. Both polypeptides were shown to be secreted extracellular proteins. The role of the first described LOX isoenzyme was studied by inactivating its gene in mice. Most Lox-/- embryos died at the end of gestation, and the few live-born pups were cyanotic and died within a few hours, autopsy revealing large aortic aneurysms. Light microscopy demonstrated structural abnormalities in the aortic walls of Lox-/- embryos, and further analysis by electron microscopy showed highly fragmented elastic fibers, discontinuity in the smooth muscle cell layers, and endothelial cell damage. Doppler ultrasonography of Lox-/- embryos in utero revealed multiple signs of cardiovascular dysfunction, which contributed to the early death of the Lox-/- mice. The results indicate that Lox has an essential role in the development and function of the cardiovascular system and that this role cannot be replaced to any significant extent by other lysyl oxidase isoenzymes.