Lymphoblastic leukemia is a type of leukemia characterized by the rapid proliferation and accumulation of lymphoblasts in the blood and bone marrow. Lymphoblasts are the most immature form of lymphocytes, a subtype of white blood cells (also known as leukocytes), in the bone marrow. The occurrence of this rapid increase is a result of certain genetic mutations. The term "acute" is used to indicate that this type of leukemia presents suddenly, progresses rapidly if left untreated, and can potentially be fatal within months.