C4A and systemic lupus erythematosus: Rare cases of monogenic form of SLE (OMIM 152700) have been reported in patients harboring mutations in TREX1 (autosomal dominant (AD)), SAMHD1 (AD), ACP5 (autosomal recessive (AR), discussed later), DNase1 (AD), DNase1L3 (AR), protein kinase C δ (PRKCD) (AR) and complement deficiency of C1q/r/s, C4 subunits (AR).