Though it is the most common cancer among children and teens, childhood leukemia is a rare disease. So says the American Cancer Society, which notes that roughly 75 percent of leukemias among children and teens are acute lymphocytic leukemia, or ALL. When a child has ALL, his or her leukemia begins to form in the lymphoid cells of the bone marrow. ALL is most common among children between the ages of two and four and is more common in boys than girls. Acute myelogenous leukemia, or AML, is another type of acute leukemia and accounts for much of the other cases of leukemia in children. AML starts in the myeloid cells where white and red blood cells and platelets are formed. In rare instances, a child may have a hybrid type of leukemia, often referred to as a "mixed lineage leukemia," where cells have features of both ALL and AML. Children with this hybrid form of leukemia are often treated as if they have ALL, and that course of treatment is typically effective. While many adult cancers are linked to lifestyle choices or environmental risk factors, childhood cancers do not have a strong link to such factors. Many childhood cancers can be traced to gene changes inside cells, and these changes often occur early in life or even before a child is born.