AFF4 and acute lymphoblastic leukemia: The AFF (AF4/FMR2) family of genes includes four members: AFF1 (or AF4, acute lymphoblastic leukemia-1 fused gene from chromosome 4), AFF2 (or FMR2, Fragile X mental retardation 2), AFF3 and AFF4 (or AF5Q31, acute lymphoblastic leukemia-fused gene from 5q31).