In murine models of ANCA-associated vasculitis, factor B–deficient mice fail to develop ANCA-induced glomerulonephritis, whereas blockade of C5 or its receptor C5aR markedly attenuates glomerular necrosis and crescent formation, underscoring the indispensable role of the alternative pathway and C5a signaling in disease pathogenesis [216–218]. The gene discussed is C5AR1; the disease is anti-neutrophil cytoplasmic antibody-associated vasculitis.