Cancer starts when a gene or several genes mutate and create cancerous cells. These cells generate cancer clusters or tumours. Cancerous cells may break away from tumours, using your lymphatic system or bloodstream to travel to other areas of your body. (Healthcare providers call this metastasis.)

For example, a tumour in your breast may spread to your lungs, making it hard for you to breathe. In some types of blood cancer, abnormal cells in your bone marrow make abnormal blood cells that multiply uncontrollably. Eventually, the abnormal cells crowd out normal blood cells.

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Credit: National Cancer Institute, USA (https://www.cancer.gov/about-cancer/causes-prevention/genetics)

Genetic changes can lead to cancer if they alter how your cells grow and spread. Most cancer-causing DNA changes occur in genes, which are sections of DNA that carry the instructions to make proteins or specialized RNA such as microRNA.  

For example, some DNA changes raise the levels of proteins that tell cells to keep growing. Other DNA changes lower the levels of proteins that tell cells when to stop growing. And some DNA changes stop proteins that tell cells to self-destruct when damaged.

For a healthy cell to turn cancerous, scientists think that more than one DNA change has to occur. People who have inherited a cancer-related genetic modification need fewer additional changes to develop cancer. However, they may never create these changes or get cancer.

As cancer cells divide, they acquire more DNA changes over time. Two cancer cells in the same tumour can have different DNA changes. In addition, every person with cancer has a unique combination of DNA changes in their cancer. 

A family cancer syndrome, also called a hereditary cancer syndrome, is a rare disorder in which family members have a higher-than-average risk of developing a certain type or types of cancer. Family cancer syndromes are caused by inherited genetic variants in certain cancer-related genes. With some family cancer syndromes, people tend to develop cancer at an early age or have other noncancerous health conditions. 

But not all cancers that appear to “run in families” are caused by family cancer syndromes. A shared environment or habits, such as exposure to air pollution or tobacco use, may cause the same kind of cancer among family members.

Also, multiple family members may develop common cancers, such as prostate cancer, just by chance. Cancer can also run in the family if family members have a combination of many genetic variants with minimal cancer risk.