Natural killer cells, or NK cells, are a part of the innate immune system which does not directly attack invading microbes. Instead, NK cells destroy compromised host cells, such as tumor cells or virus-infected cells. It recognises such cells by a condition known as "missing self". This term describes cells with low levels of a cell-surface marker called MHC I (major histocompatibility complex). This can occur in viral infections of host cells. They were named "natural killer" because they do not require activation in order to kill cells that are "missing self".