C1q deficiency is a rare disorder associated with recurrent skin lesions, chronic infections, systemic lupus erythematosus (SLE) or SLE-like diseases. It has also been associated with a kidney disease known as mesangial proliferative glomerulonephritis. C1q is a complex and together with other proteins, C1r and C1s, it forms the C1 complex. This complex is important for the activation of the complement system (a group of proteins that work with the immune system). It also disposes cells that are dead. C1q deficiency presents in 2 different forms, absent C1q protein or abnormal C1q protein. Symptoms include infections (ear infections (otitis media), meningitis, urinary tract infections, oral infections); skin lesions (small blisters (vesicles), dark patches, and atrophic areas) that get worse upon light exposure; cataracts; loss of eyelashes, eyebrows, and scalp hair; blood in urine; and glomerulonephritis. About 93% of cases are associated with systemic lupus erythematosus. It can be caused by mutations in the C1QA, C1QB or C1QC genes and is inherited in an autosomal recessive pattern. Treatment depends on the symptoms. Recently, it was shown that C1q production can be restored by allogeneic hematopoietic stem cell transplantation, a procedure in which a person receives blood-forming stem cells (cells from which all blood cells develop) from a genetically similar, but not identical donor. The disease is Monarch Disease Ontology identifier MONDO_0013343 (C1Q deficiency). Also known as: C1QD, C1q deficiency.