What Causes Cancer?

February 12, 2026

This page was reviewed under our medical and editorial policy by Heather Hampel, M.S., C.G.C., professor, Department of Medical Oncology & Therapeutics Research, City of Hope^® Cancer Center Duarte.

Whether influenced by genetics, environmental factors or lifestyle factors, cancer develops when abnormal cells grow and divide uncontrollably, caused by either acquired or inherited changes (mutations) to the DNA within cells.

Mutations occur frequently in DNA, but normally cells correct these errors. When they don’t, a genetic mutation can throw a healthy cell off course, leading to:

• Rapid growth of new cells with the same mutation
• Failure to stop uncontrolled cell growth
• Mistakes when repairing DNA errors

While these mutations are the most common, many more may influence cells to become cancerous. Widespread research has also shown that certain factors may increase cancer risk.

What Types of Gene Mutations Cause Cancer?

Cancer develops when there are changes, or mutations, to the DNA in a cell’s genes. It’s important to understand the difference between gene mutations a person is born with (inherited) and gene mutations that occur after birth (acquired).

Inherited gene mutations: These are passed down from parent to child and exist in every cell of a person’s body. They account for 10% to 15% of cancers. Genetic tests look for inherited changes in a person’s genes that may increase the risk of cancer. It’s important to note that people who inherit a gene mutation don’t inherit cancer, but instead inherit an increased risk of developing cancer. It is also important to note that most cancers are not hereditary.

Acquired gene mutations: These develop during a person’s lifetime for various reasons, including environmental or lifestyle factors that damage a cell’s DNA. They also occur by chance during the aging process. They are a common cause of cancer. It is common for oncologists to order genetic testing on a patient’s tumor to look for the gene mutations that caused the tumor to develop, because this can sometimes give them options for targeted therapies. This is different from genetic testing performed on blood or saliva to look for inherited gene changes that someone was born with and can pass on to their children.

Some of the genes that can cause cancer include:

• Oncogenes: These are altered forms of normal genes that control cell growth and division. When mutated, they mistakenly send nonstop signals that direct cells to grow and divide rapidly, ultimately forming tumors.
• Tumor suppressor genes: These are responsible for keeping cell division under control or killing cells at the right time. If these genes are turned off due to mutation, cells may grow uncontrollably.
• DNA repair genes: These help fix mistakes in a cell’s DNA. When these genes are mutated, they cannot repair these mistakes and they may build up inside the cell, leading to rampant cell growth. DNA repair genes are a type of tumor suppressor gene.

Cancer Risk Factors

Research over decades has found that certain controllable and uncontrollable risk factors may increase a person’s risks for developing cancer. While scientists continue to explore the complexities surrounding what causes cancer, it’s important to understand the most well-known and suspected risk factors of cancer, including:

Age: With age comes more exposure to carcinogens and inflammatory processes. In addition, harmful gene changes that occur by chance can build up over time. That’s why cancer is mostly diagnosed in people 65 or older.

Alcohol: Alcohol consumption is the third leading preventable cause of cancer in the United States, increasing risk for at least seven types of cancer. Consuming more than a glass a day for women or more than two glasses a day for men increases cancer risk.

Autoimmune diseases: Certain autoimmune disorders, like Crohn’s disease and rheumatoid arthritis, cause chronic inflammation, which may damage cell DNA and lead to uncontrolled cell growth and cancer.

Chemicals: Exposure to carcinogens in the home, workplace, and outdoors — such as asbestos, radon, pollution, pesticides, fertilizers, herbicides, insecticides and toxins like vinyl chloride or polychlorinated biphenyls (PCBs) — may put people at increased risk for developing cancer.

Family history: Approximately 10% to 15% of cancer cases are due to genetics or inherited conditions, such as Lynch syndrome (due to mutations in genes such as MLH1, MSH2 (EPCAM), MSH6, or PMS2, or Hereditary Breast-Ovarian Cancer syndrome (due to mutations in the BRCA1 or BRCA2 genes).

Infections or viruses: Helicobacter pylori (H. pylori) infection, Epstein-Barr virus (EBV, mononucleosis or “mono”) and human papillomavirus (HPV), for example, may raise cancer risk by damaging DNA, causing chronic inflammation or suppressing the immune system.

Lack of physical activity: Research from the American Cancer Society and the National Cancer Institute links exercise with a lower risk of 13 specific types of cancer by helping control weight and strengthening the immune system.

Obesity: Obesity raises the risk of breast, colorectal, endometrial, esophageal, kidney, pancreatic and kidney cancers, among others, because excess fat cells produce more estrogen and insulin, which are hormones that fuel cancer growth.

Poor eating habits: A diet high in fat, processed foods, red meat, artificial sweeteners, refined carbohydrates and acrylamide increases cancer risk.

Radiation: Cancer is associated with exposure to radiation from medical imaging or radiation therapy for cancer treatment, or from nuclear or radon gas, sometimes present in soil and possibly in your home.

Sunlight and UV rays: Sun exposure or other excess exposure to ultraviolet (UV) rays, including from tanning beds, may cause skin damage or skin cancer.

Tobacco: Smoking is the number one cause of lung cancer, causing about 90% of cases. Exposure to second-hand smoke and chewing tobacco also increases cancer risk. Tobacco also significantly increases risks for cancers of the mouth, throat, bladder, kidney, pancreas, esophagus, stomach, liver, cervix and colon, as well as acute myeloid leukemia.

While not all cancer is avoidable, an estimated 42% of cancer cases and 45% of cancer deaths in the United States are attributed to modifiable risk factors — a powerful reason to make lifestyle changes that reduce cancer risk.