Acute lymphoblastic leukemia (ALL) is the most common cancer in children and represents about one-quarter of all cancers among persons younger than 15 years. Acute lymphoblastic leukemia is a cancer of the blood and bone marrow (spongy tissue in the center of bone). In ALL, too many bone marrow stem cells develop into a type of white blood cell called lymphocytes. These abnormal lymphocytes are not able to fight infection very well. Also, as the number of these lymphocytes increases, there is less room for healthy white blood cells, red blood cells, and platelets. This may lead to infection, fatigue, and easy bleeding. The January 28, 2009, issue of JAMA includes an article about genetic variations associated with treatment response in childhood ALL. This Patient Page is based on one previously published in the March 21, 2007, issue of JAMA.