Genetics Northern California

Cancer Genetics

All cancer is caused by genetic changes that happen over time in the cells of the body. Cancer starts when a cell builds up enough genetic errors (mutations) to interfere with normal cell growth. The cell begins to grow uncontrollably and is able to spread to other parts of the body. This is a long process that involves many different mutations and takes many years. For most cancers, all of the genetic errors happen after a person is born. They are the result of aging and exposure to things in the environment. Mutations build up in cells similar to the way mistakes can be introduced into a long report that gets retyped many times.

A small percentage of cancer is related to cancer predisposition genes. In this situation, certain genetic instructions (genes) have errors already present when they are passed on from parent to child. That means all the cells in the body start with this mutation. When every cell already has one genetic error, it makes it much more likely that at least one cell in the body will eventually become cancerous. This is an active area of research as we try to understand which genes are most likely to have inherited mutations. Cancer predisposition genes have been identified for many different types of cancer, including breast and ovarian cancer, colon cancer, uterine cancer, kidney cancer, and medullary thyroid cancer. It is estimated that about 5% to 10% of common cancers are related to a hereditary cancer predisposition gene.  

Families who have a mutation in a cancer predisposition gene are more likely to have:

  • Several people in the family with similar or related cancers

  • Cancer affecting more than one generation of the family

  • Cancer that starts at an earlier age than average (often in the 30's or 40's)

  • Individuals who have had more than one cancer (For example:  both colon and uterine cancer, or cancer in both breasts)

How common is it to have several family members with cancer?
Most people have cancer in their family history. This is because cancer is very common: about one in three Americans develops some type of cancer during their lifetime. Cancer is the second leading cause of death for adults in this country. So, it should come as no surprise that many families have more than one family member with cancer.

When several people in a family have the same type of cancer, it can be hard to know exactly why it happened. Sometimes it is just “bad luck” or perhaps better described as coincidence. Some cancers, such as breast and prostate, are so common that it is not unusual for someone to have two affected relatives just by chance. Multiple cases of cancer might also be related to a common lifestyle or shared environment. It is likely that some families have minor genetic differences that make them more sensitive to things in the environment, causing a slightly higher chance of developing cancer.

In a small number of families, though, multiple cases of cancer happen because an inherited mutation causes a much higher risk for cancer. 


What should I do if I am worried about having an inherited predisposition to cancer? 
If you have concerns about your personal or family history of cancer, talk to your doctor or nurse practitioner. They can help you decide if an appointment in Genetics would be useful. If you have multiple cases of breast and/or ovarian cancer, or multiple cases of colon cancer or uterine cancer in your family and would prefer to speak directly with someone in Genetics, call your closest Kaiser Permanente Genetics Department.  
Gather your family history using our family history form.  You may want to consider collecting medical records (especially pathology reports) from people in your family who have had cancer. It may be harder to get records as the years pass, so it is usually better to take this step sooner rather than later.  These can be useful to you in the future, and are often important in a genetic evaluation of your family.

To learn more about genetic testing in a family: Hereditary Cancer Risk: Who should be tested first?


Last reviewed: November 30, 2017
Reviewed by: Kimberly Barr, MS LCGC