The jo-1 antigen is histidyl-transfer ribonucleic acid (tRNA) synthetase which aids in the incorporation of histidine residues into proteins. Anti-Jo-1 autoantibodies are found in autoimmune connective tissue diseases, and they bind to conformational epitopes of the enzyme protein and inhibit its catalytic activity in vitro. Jo 1 antibodies are a marker for polymyositis and occur most commonly in myositis patients who also have interstitial lung disease.