AFF1 and AFF4 share some conserved functional domains in their protein structures.24,41 Both of them are reported to form fusion genes with the mixed lineage leukemia (MLL) gene, which is associated with leukemia.32,42 Both AFF1 and AFF4 act as partners of P-TEFb to regulate transcription elongation. The gene discussed is AFF1; the disease is leukemia.