AFF1 and acute lymphoblastic leukemia: However, infants with the t(4;11) translocation between KMT2A and AFF1, giving rise to KMT2A::AFF1 fusion (classically known as MLL-AF4), have a higher associated risk [2, 4] and, importantly, represent a vast majority of pro-B-ALL patients diagnosed at ≤ 1 year of age [4, 8].