Advances in biology and novel treatments of SCLC: The four-color problem in uncharted territory.

Treatment for small cell lung cancer (SCLC) has not changed significantly compared to the overwhelming development of targeted therapies for non-small cell lung cancer. However, recent epigenetic and expressional analyses have revealed that SCLC can be divided into four distinct subtypes, which may lead to precision treatments. The situation appears slightly similar to the "four-color problem," a classic mathematical problem stating that no more than four colors are required to color the regions so that no two adjacent areas have the same color. This review introduces the framework for subtyping SCLC into four molecular subtypes and the promising targeted treatment for each subtype.