Childhood leukemia is thought to be associated with genetic factors, and investigators have focused on leukemia-promoting factors in the past. However, the development of leukemia could be a result of the inability of the immune system to attack and eliminate upcoming malignant cells (A). In the presence of KIR genes, the risk of childhood leukemia is reduced because of a possible attack of leukemia cells by NK cells or T cells (B). Inheriting a higher number of activating KIR genes (≥ 4) is associated with significant reductions in risk for ALL in children (C).