Viral co-transduction of the MLL-AF4 targets (Zeisig et al., 2004) HOXA9 and MEIS1 is sufficient to cause acute leukemia in mouse bone marrow progenitors, arguing that these transcription factors represent a major etiologic pathway in MLL-r leukemia (Corral et al., 1996; Chang et al., 2010a; Jo et al., 2011; Kroon et al., 1998; Calvo et al., 2002). Here, AFF1 is linked to leukemia.