To investigate the roles of MASP-1 and MASP-3 in lupus, we generated Masp1 gene knockout lupus-prone MRL/lpr mice (Masp1/3−/− MRL/lpr mice), lacking both MASP-1 and MASP-3, and analyzed their renal disease. Here, MASP1 is linked to systemic lupus erythematosus.