A new biomarker, a peptide having sequence SSKITHRIHWESASLLR*, wherein the side chain of the C-terminal arginine denoted with the asterisk is lacking the NH2- C═NH moiety normally present in the side chain. Usefulness of the biomarker in the diagnosis of neurological and/or neuropsychiatric disorders (in particular autism) is disclosed, as well as are methods for determining the concentration of the new biomarker and antibodies directed to the new biomarker. Treatment of autism, comprising administering a complement factor I inhibitor to the subject.
