Tumor markers are substances, usually  proteins that are produced by the body in response to cancer growth or by the cancer tissue itself. Some tumor markers are specific for one type of cancer, while others are seen in several cancer types. Many of the well-known markers are seen in non-cancerous conditions as well as cancer. The goal is to be able to screen for and diagnose cancer early, when it is the most treatable and before it has had a chance to grow and spread. For some types of cancer, they provide additional information that can be considered in conjunction with a patient’s medical history and physical exam as well as other laboratory and/or imaging tests.