This could be relevant since Cybb heterozygous (Cybb+/–) female MRL.Faslpr mice, in which 50% of cells lacked CYBB protein due to mosaic X inactivation, developed exacerbated SLE-like disease (20), indicating that the presence of a subset of cells that lacks CYBB can have a dominant effect in promoting the disease. The gene discussed is PSMB5; the disease is systemic lupus erythematosus.