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.
More top news
It could be the world's biggest open air trampoline. The bizarre project has the backing of London mayor Boris Johnson.
This is the moment a teenage wakeboarding champion was pulled up a river at 84mph by a £1 million Ferrari.
Arsenal host Borussia Dortmund at the Emirates this evening as they strive to secure a place in the final 16 of the Champions League.