For a mutation to be passed from one generation to the next the mutation must occur in a germ-cell that will produce eggs or sperm. If a mutation occurs in a non-germ-cell the mutation can't be passed to future generations. However, the mutation will be passed to descendants of the cell in which it occurred and as a result the population of tumour cells in the body will progressively increase. Some types of breast cancer are highly heritable-there is a greater than 90% chance that it will occur in many generations. However, the heritable forms of breast cancer make up a very small percentage of all breast cancer cases. DNA damage often occurs during mitosis so there are systems in place to repair the damage. Mutations that affect the ability of those systems to do their job can be passed from generation to generation and increase the risk of cancer of any type occurring in a family.