To become a bone marrow donor, you have to be a blood a donor first, so that your blood can be checked and you must be aged between 18 and 49 years old.
You can join when you next give blood, or at the same time as your first donation. They will then check that there is no medical reason preventing you from being both a blood donor and a stem cell donor.
At the time of your blood donation they will take an extra blood sample, so that they can identify your tissue type for the registry from your DNA - the genetic material our bodies are made up from.
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Officers were given the touching letter at Brixton Station while they were on a reassurance patrol after yesterday's attack
The 52-year-old had previous convictions for violence but was not on the anti-terror watchlist.
In a statement Charlton Athletic said: Keith was a true hero who will be greatly missed by all the Charlton family and everyone at the club.