Introduction Cancers are often first classified by their tissue of origin, but there are several types of cancer for each tissue. Further, each of these can have several subdivisions. For example, head and neck cancers can be further divided into seven cancers, including head and neck squamous cell carcinoma (HNSC). HNSC itself has six subtypes, too. This hierarchy can be represented in a directed acyclic graph (DAG), as shown below.