When someone inherits an abnormal copy of a gene, their cells already start out with one mutation.
Often this is not a problem, as the other gene copy is still working.
These people might be more likely to get cancer if they smoke than someone who does not have these gene changes.
Less often, cancers in a family are strongly linked to an inherited gene mutation that is part of a family cancer syndrome.
But in some cases the cancer is caused by an abnormal gene that is being passed along from generation to generation.