Cancer is caused by mutant cells. These mutations can be caused by exposure to toxins, or mistakes in DNA replication. The tendency to develop a certain type of cancer can also be inherited. We all have some mutations in our DNA, but not all of them cause problems. It is only bad if the mutation happened in a gene in a way that changes the information it codes for. This could make the cells not work properly. In the case of cancer, it could make cells divide uncontrollably and cause a tumor. Luckily, having one mutation doesn’t guarantee a person will get cancer. The cells manage to work okay unless a second mutation occurs that helps out the first one. And just because a parent gets cancer doesn’t mean that his/her offspring will too. Probably less than 15% of cancers are due to an inherited predisposition.