A homozygous loss-of-function C1S mutation is associated with Kikuchi-Fujimoto disease

•Kikuchi-Fujimoto disease (KFD) is a self-limited inflammatory disease of unknown cause.•Possible defects in the classical complement pathway have been identified in some cases of KFD.•We describe a novel C1S loss-of-function mutation in a patient with KFD.•This description might supports an action of complement in KFD pathogenesis.