Histone demethylase that specifically demethylates 'Lys-27' of histone H3, thereby playing a central role in histone code (PubMed:17851529, PubMed:17713478, PubMed:17761849). Demethylates trimethylated and dimethylated but not monomethylated H3 'Lys-27' (PubMed:17851529, PubMed:17713478, PubMed:17761849). Plays a central role in regulation of posterior development, by regulating HOX gene expression (PubMed:17851529). 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 (PubMed:17761849). Plays a demethylase-independent role in chromatin remodeling to regulate T-box family member-dependent gene expression (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 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 (PubMed:17500065, PubMed:17713478). Interacts with SUPT6H. Interacts with SMARCA4 (By similarity). The disease is caused by variants affecting the gene represented in this entry. Escapes X chromosome inactivation. Belongs to the UTX family.