Cancer is not a single disease but includes many different diseases. Cancer is caused by abnormal cell growth resulting from mutations in the DNA. These mutations are induced by mutagenic substances such as smoke, ionizing radiation and pollutants. Viruses are also highly cancerogenic because they infect their host by inserting DNA into the host genome. Mutations and DNA insertions lead to altered gene expression. Cancer develops if cells acquire an entire set of these mutations and start growing uncontrollably.

In healthy cells so-called tumor suppressor genes function to prevent uncontrolled cell growth. If the function of these genes is disrupted, this can lead to the development of abnormal cell growth and the development of cancer. Mutations in these genes can be inherited, meaning that some cancers have a strong heritable element.

Proto-oncogenes are positive cell-cycle regulators. When mutated, proto-oncogenes can become oncogenes and cause cancer. Overexpression of the oncogene can lead to uncontrolled cell growth. This is because oncogenes can alter transcriptional activity, stability, or protein translation of another gene that directly or indirectly controls cell growth.

There are over 100 different known types of cancer in humans. These tumor cells form lumps and invade other organs. Cancer causes about 15% of all human deaths, and cost society an estimated 1.2 trillion USD. Cancer rates are increasing, as more people live longer.

Diagram showing a cross section of a blood vessel. A tumor is attached to the exterior of the blood vessel. A piece of the tumor enters the blood vessel and travels through it, then travels out of the blood vessel at a second location and begins to form a second tumor.

Figure 1. An illustration showing how primary cancer invades through the blood vessel producing secondary cancer (or metastasis). This image was provided by Jane Hurd and released by the National Cancer Institute.