Histone demethylase that specifically demethylates 'Lys- 4' of histone H3, thereby playing a central role in histone code. Does not demethylate histone H3 'Lys-9', H3 'Lys-27', H3 'Lys-36', H3 'Lys-79' or H4 'Lys-20'. Demethylates trimethylated and dimethylated but not monomethylated H3 'Lys-4'. May stimulate transcription mediated by nuclear receptors. May be involved in transcriptional regulation of Hox proteins during cell differentiation. May participate in transcriptional repression of cytokines such as CXCL12. Interacts with SUZ12; the interaction is direct. Interacts with the viral protein-binding domain of RB1. Interacts with ESR1, MYC, MYCN and LMO2. Belongs to the JARID1 histone demethylase family. 2 isoforms of the human protein are produced by alternative splicing. Note: This description may include information from UniProtKB.