Chromatin reader component of the super elongation complex (SEC), a complex required to increase the catalytic rate of RNA polymerase II transcription by suppressing transient pausing by the polymerase at multiple sites along the DNA (PubMed:20159561, PubMed:20471948, PubMed:25417107, PubMed:27105114, PubMed:27545619). Specifically recognizes and binds acylated histone H3, with a preference for histone H3 that is crotonylated (PubMed:25417107, PubMed:27105114, PubMed:27545619, PubMed:30374167, PubMed:30385749). Crotonylation marks active promoters and enhancers and confers resistance to transcriptional repressors (PubMed:25417107, PubMed:27105114, PubMed:27545619). Recognizes and binds histone H3 crotonylated at 'Lys-9' (H3K9cr), and with slightly lower affinity histone H3 crotonylated at 'Lys-18' (H3K18cr) (PubMed:27105114). Also recognizes and binds histone H3 acetylated and butyrylated at 'Lys-9' (H3K9ac and H3K9bu, respectively), but with lower affinity than crotonylated histone H3 (PubMed:25417107, PubMed:27105114, PubMed:30385749). In the SEC complex, MLLT3 is required to recruit the complex to crotonylated histones (PubMed:27105114, PubMed:27545619). Recruitment of the SEC complex to crotonylated histones promotes recruitment of DOT1L on active chromatin to deposit histone H3 'Lys-79' methylation (H3K79me) (PubMed:25417107). Plays a key role in hematopoietic stem cell (HSC) maintenance by preserving, rather than confering, HSC stemness (PubMed:31776511). Acts by binding to the transcription start site of active genes in HSCs and sustaining level of H3K79me2, probably by recruiting DOT1L (PubMed:31776511). Crotonylated lysine binding is strongly inhibited by the peptide XL-07i, carrying a 2-furancarbonyl side chain and capped with a hydrophobic carboxybenzyl group (PubMed:30374167). XL-07i targets the unique pi-pi-pi stacking interaction at the crotonylation recognition site (PubMed:30374167). Component of the super elongation complex (SEC), at least composed of EAF1, EAF2, CDK9, MLLT3/AF9, AFF (AFF1 or AFF4), the P-TEFb complex and ELL (ELL, ELL2 or ELL3) (PubMed:20159561, PubMed:20471948, PubMed:22195968, PubMed:23260655, PubMed:25417107). Interacts with BCOR (PubMed:10898795). Interacts with CBX8 (PubMed:11313972). Interacts with ALKBH4 (PubMed:23145062). Colocalizes with acylated histone H3 (PubMed:25417107, PubMed:27105114). Colocalizes with histone H3 crotonylated at 'Lys-18' (H3K18cr) (PubMed:27105114). Enriched in undifferentiated hematopoietic stem cells in fetal liver, cord blood and bone marrow. The YEATS domain specifically recognizes and binds acylated histones, with a marked preference for histones that are crotonylated (PubMed:27105114, PubMed:27545619). Also binds histone H3 acetylated at 'Lys-9' (H3K9ac), but with lower affinity (PubMed:25417107, PubMed:27105114). Binds crotonylated lysine through a non-canonical pi-pi-pi stacking mechanism (PubMed:30374167, PubMed:30385749). The YEATS domain also binds DNA (PubMed:30385749). A chromosomal aberration involving MLLT3 is associated with acute leukemias. Translocation t(9;11)(p22;q23) with KMT2A/MLL1. The result is a rogue activator protein. Fusion protein KMT2A-MLLT3 interacts with MEN1 and PSIP1 (PubMed:22936661, PubMed:25305204). A chromosomal aberration involving MLLT3 was observed in a patient with neuromotor development delay, cerebellar ataxia and epilepsy. Translocation t(4;9)(q35;p22).