In any complex machine, the lack of a single part can lead to big trouble. That is the problem faced by the 170,000 people globally who have the bleeding disorder known as haemophilia. A genetic mutation (usually inherited) suppresses the production of proteins that make blood coagulate (see page S158). Internal bleeding into the joints causes bone degradation and excruciating pain (S170), and even mild injuries can be life-threatening.
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