PNH stands for paroxysmal nocturnal haemoglobinuria. It is a rare and debilitating disease of the bone marrow that affects the blood and major organs. PNH is not hereditary, but rather is an acquired illness caused by injury to the bone marrow stem cell. Put very simply, PNH occurs when a gene required for the bone marrow to produce normal blood cells becomes mutated, resulting in defective blood cells. Once affected, the mutated bone marrow gene produces blood cells – red, white and platelets – that do not function as effectively as normal cells. People living with PNH have red blood cells that lack a protein that protects them from being destroyed by complement – a substance in the blood that helps the immune system stave off foreign bodies. Without this protein, red blood cells are destroyed by complement in a process called haemolysis, much more quickly than in healthy individuals. Haemoloysis, or the destruction of red blood cells, can result in a number of health complications. PNH patients can experience symptoms including anaemia (lowered haemoglobin levels), liver and/or kidney failure, heart attack, stroke and blood clots.