C5 is the éminence grise of the complement cascade’s druggable proteins, and the target of effective therapies for diseases with pathogenic complement dysregulation, of which paroxysmal nocturnal haemoglobinuria (Rother et al., 2007) and atypical haemolytic uraemic syndrome (Nürnberger et al., 2009) are notable examples. The gene discussed is C5; the disease is paroxysmal nocturnal hemoglobinuria.