Leukemia, sometimes called leukaemia, is a kind of blood cancer that generally begins in the bone marrow and results in an abnormally large number of blood cells. These undeveloped blood cells are known as "blasts" or "leukemia cells." The term "leukemia" comes from Greek words meaning "change to white" because of the abnormal blood cells' appearance under the microscope. The cause of leukemia is not clear but factors such as genetics, environment, and infection may play a role. Leukemia can be classified by how many types of blood cells are affected. If it is limited to one type of cell (such as lymphocytes or monocytes) then it is called a single-cell leukemia. If it affects two different types of cells (such as lymphocytes and myeloid cells) then it is called a double-cell leukemia. Triple-cell leukemias involve all three types of blood cells. The most common form of leukemia is chronic myelogenous leukemia (CML), which means "chronic malignancy of the blood-making machinery" due to its similarity with other chronic diseases that affect the blood-making process such as HIV/AIDS or hepatitis B and C. It is also called Philadelphia chromosome positive (Ph+) or BCR-ABL positive. This means that there is an error in the genetic code for some of the proteins made by red blood cells that causes them to become malignant.