PRKCD and systemic lupus erythematosus: The lupus ERK pathway signaling defect was traced to PKCδ inactivation caused by nitration, a form of oxidative damage caused by superoxide (O2−) combining with nitric oxide (NO), an intracellular signaling molecule, to form peroxynitrite (ONOO−), which nitrates tyrosine, and the inactive PKCδ fraction in lupus T cells is nitrated [30].