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What causes breast cancer?

Breast cancer is a complex disease with no single cause. Scientific evidence shows that a combination of internal and external factors influences a person’s risk of developing breast cancer.

Risk factorsRisk factor:
Anything that increases a person’s chance of developing a disease, such as cancer.
are things that can increase your risk for developing a specific disease and are established by conclusive research. However, they cannot be seen as rules or guarantees about whether or not a person will develop breast cancer. Even if you have several risk factors for breast cancer, this does not mean you will develop the disease. On the other hand, some women who are diagnosed with breast cancer have no identifiable risk factors other than being a woman.

Risk Factors for Breast Cancer

Non-modifiable risk factors

There are some risk factors for breast cancer that we can’t change – these are known as non-modifiable risk factors. Inherited causes of breast cancer are an example of a non-modifiable risk factor. They are part of the geneticGenetic:
Related to or caused by the genes.
makeup we received from our parents and do not change through life. We have genesGene:
A segment of DNA that contains hereditary information.
that help protect against the development of cancer, including breast cancer. Two of these genes, known as BRCA1BRCA1:
A gene which, when damaged (mutated), places a person at greater risk of developing breast and/or ovarian cancer, compared to someone who does not have the mutation.
A gene which, when damaged (mutated), places a person at greater risk of developing breast and/or ovarian cancer, compared to someone who does not have the mutation.
, play an important role in protecting against breast and ovarian cancer by helping to repair damaged DNADNA:
Short for Deoxyribonucleic acid. DNA is found in the cells of our bodies. It contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. Your DNA contains genetic information that is unique to you.
that could lead to uncontrolled cell growth. When BRCA1 or BRCA2 genes contain a harmful mutationMutation:
Any change in the DNA sequence of a cell. Mutations can be harmful, beneficial, or have no effect. Certain mutations may lead to cancer or other diseases.
, DNA repair does not occur as it should.

A person inherits one copy of these genes from each of their parents. This means a mutated BRCA gene can be passed down from either a person’s mother or father, or both. It also means that a man can inherit a mutated BRCA gene, increasing his risk for developing breast cancer.

Although most women are at average risk for breast cancer, a small number (1-2%) are at high risk. Approximately 5-10% of breast cancers in Canada are the result of inherited or genetic factors. For women whose family history suggests that they may be at high risk for breast cancer from hereditaryHereditary:
A trait, for example eye colour, that is transmitted genetically from one generation to the next. Some genetic mutations are inherited, for example mutations of the BRCA 1 or BRCA 2 genes that lead to a strong increase in the risk of breast cancer.
factors, Genetic counsellingGenetic counsellor:
A health care provider with specialized training and experience in the areas of medical genetics and counselling.
and testing are available to confirm BRCA1, BRCA2, and other known genetic mutations that increase breast cancer risk.

Other risk factors for breast cancer which we may have less influence over include the age when you start your period and go into menopause, whether you have children, and if you are able to breastfeed them.

Modifiable risk factors

Modifiable risk factors for breast cancer are factors that can be changed. These are factors that we have more influence over and can affect us throughout our lives. Modifiable risk factors include our lifestyle and the environment we live in, for example, our level of physical activity, how much alcohol we drink, our use of hormone replacement therapy and our exposure to chemicals that are known to change cellCell:
The basic structural and functional unit of all organisms.

We are able to influence our level of exposure to many of these factors. By learning about them, you can take steps to reduce your breast cancer risk.

More research is needed in this area to understand better how risk factors work together to result in the cell DNA changes that lead to breast cancer. Research helps to improve our knowledge of breast cancer, giving us a stronger understanding of breast cancer risk and ways to try to reduce the risk. 


National Cancer Institute. BRCA1 and BRCA2: Cancer Risk and Genetic Testing. Accessed January 1, 2014.