FAS and systemic lupus erythematosus: Therefore, we studied the role of MMP-9 in SLE by comparing the lpr loss-of-function mutation in the apoptosis mediator fas knockout mice on a C57Bl/6 background (LPR−/− mice) with mice lacking both MMP-9 and functional apoptosis receptor Fas (LPR−/−MMP-9−/− mice).