Histone demethylase that specifically demethylates 'Lys-27' of histone H3, thereby playing a central role in histone code. Demethylates trimethylated and dimethylated but not monomethylated H3 'Lys-27'. Plays a central role in regulation of posterior development, by regulating HOX gene expression. Demethylation of 'Lys-27' of histone H3 is concomitant with methylation of 'Lys-4' of histone H3, and regulates the recruitment of the PRC1 complex and monoubiquitination of histone H2A (By similarity). Plays a demethylase-independent role in chromatin remodeling to regulate T-box family member-dependent gene expression (PubMed:21095589). 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 Component of the MLL2/3 complex (also named ASCOM complex), at least composed of KMT2D/MLL2 or KMT2C/MLL3, ASH2L, RBBP5, WDR5, NCOA6, DPY30, KDM6A (or KDM6B), PAXIP1/PTIP, PAGR1 and alpha- and beta-tubulin (By similarity). Interacts with TLE1 (PubMed:9854018). Interacts with SUPT6H (PubMed:23503590). Interacts with SMARCA4 (PubMed:21095589). Expressed in brain, heart and spleen. Widely expressed at 13.5 dpc. Escapes X chromosome inactivation. Belongs to the UTX family.