Functional characterisation at the level of individual complement pathways identified K57 as a novel C5 inhibitor, which is a fully efficacious inhibitor of the terminal pathway in response to both CP and AP activation, and a potential therapeutic candidate for complement-mediated disorders, such as paroxysmal nocturnal haemoglobinuria and atypical haemolytic uraemic syndrome. The gene discussed is CP; the disease is paroxysmal nocturnal hemoglobinuria.