Abstract: The present invention concerns subject matter connected to or making use of lgLON5, lgLON5 fragments and variants of lgLON5 and lgLON5-fragments. In particular the present invention relates to a use of a polypeptide comprising one or more sequences of IgLON5, an lgLON5-fragment or a variant thereof for the diagnosis of a disease, in vitro methods for diagnosing such a disease, a polypeptide comprising one or more sequences of lgLON5, an lgLON5-fragment or a variant thereof or a nucleic acid encoding said polypeptide for use in the treatment of a disease, a pharmaceutical composition comprising such polypeptide, a method for treating such a disease, an autoantibody binding to IgLON5, an lgLON5-fragment or a variant thereof, a method for isolating such autoantibody, a medical or diagnostic device comprising such autoantibody or such polypeptide and a test kit for the diagnosis of a disease, which test kit comprises such autoantibody and/or such polypeptide.