A tumor marker in a biomedical setting can be defined as a biological object present in human tissue and/or body fluids that is capable to differentiate between normal and abnormal biological conditions. The National Institutes of Health added that it should be measured objectively and is evaluated as an indicator of pathogenic processes or biological responses to a therapeutic intervention. With this definition, a wide range of characteristics can be used as a tumor marker, such as easily observable skin lesions or more inconspicuous variables such as proteins or RNA present in tissue, serum, or urine. Nowadays, the term tumor marker is inextricably linked to molecular markers.