A mutation in the LRP5 gene can make bones extremely dense and difficult to break.
Why it is true
A mutation in the LRP5 gene can make bones extremely dense and hard to break. People with this mutation rarely suffer fractures, but they may also have hearing or vision problems due to nerve compression.
Source: Nature
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.