A highly similar set of genes including AZU1, CAMP, CTSG, DEFA4, ELANE, LTF, and MPO is activated and forms part of a co-expression module in isolated low-density granulocytes (LDG) from systemic lupus erythematosus (SLE) patients and it is thought that LDGs are immature neutrophils that have been released into the circulation during granulopoiesis [45, 48]. Here, MPO is linked to systemic lupus erythematosus.