Histone demethylase that specifically demethylates 'Lys-27' of histone H3, thereby playing a central role in histone code (PubMed:17825402, PubMed:17851529, PubMed:17713478, PubMed:18003914). Demethylates trimethylated and dimethylated H3 'Lys-27' (PubMed:17825402, PubMed:17851529, PubMed:17713478, PubMed:18003914). Plays a central role in regulation of posterior development, by regulating HOX gene expression (PubMed:17851529). Involved in inflammatory response by participating in macrophage differentiation in case of inflammation by regulating gene expression and macrophage differentiation (PubMed:17825402). Plays a demethylase-independent role in chromatin remodeling to regulate T-box family member-dependent gene expression by acting as a link between T-box factors and the SMARCA4-containing SWI/SNF remodeling complex (By similarity). 2 2-oxoglutarate + N(6),N(6),N(6)-trimethyl-L-lysyl(27)-[histone H3] + 2 O2 = 2 CO2 + 2 formaldehyde + N(6)-methyl-L-lysyl(27)-[histone H3] + 2 succinate Interacts with TLE1 (By similarity). Component of the MLL4 complex, at least composed of KMT2B/MLL4, ASH2L, RBBP5, WDR5, and KDM6B (PubMed:17825402). Interacts with TBX21, SMARCA4, SMARCC1 and SMARCC2 (By similarity). By 12-O-tetradecanoylphorbol-13-acetate (TPA) in myeloid leukemia cells. The disease is caused by variants affecting the gene represented in this entry. Belongs to the UTX family. Extended N-terminus.