The present invention relates to a method for diagnosing multiple sclerosis (MS) and/or clinically isolated syndrome (CIS) and/or radiologically isolated syndrome (RIS) or a predisposition for either condition in a subject, the method comprising determining the presence of an anti-KIR4.1 antibody in a sample obtained from said subject by (a) contacting said sample with a protein; and (b) detecting the formation of a protein-anti-KIR4.1 antibody complex; wherein said protein is KIR4.1, wherein glycosylation of the large extracellular domain of said KIR4.1 is as in human oligodendrocytes or glycosylation of the large extracellular domain is absent; and wherein the formation of said complex is indicative of MS, CIS, RIS or a predisposition therefore. Furthermore provided is an antibody or fragment or derivative thereof competing with the anti-KIR4.1 antibody in a sample obtained from a patient having MS.