Genes are segments of DNA located on chromosomes. Mutations can occur over time, sometimes because of exposure to environmental factors such as smoking or viruses, and those changes can cause cells to become cancerous. As a result, increased age alone can be a risk factor for cancer: More than 75% of all cancer cases are diagnosed in people aged 55 or older. Only about 5% to 10% of cancers are genetically inherited, and those cancers tend to occur earlier in life. Oncogenes are genes that cause cancer, while tumor suppressor genes play a role in controlling or stopping cancer.