AFF1 and acute lymphoblastic leukemia: The incidence of t(4;11)(q21;q23)/MLL-AF4 positive ALL has a characteristic bimodal age distribution with a major peak in early infancy, occurring in over 50% ALL cases in infants aged less than 6 months, in 10–20% of older infants, in about 2% of children, and in almost 10% of adults [27–29].