Histone demethylase that specifically demethylates ‘Lys-4’ of histone H3, thereby playing a central role in histone code (PubMed:28262558). 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’. Participates in transcriptional repression of neuronal genes by recruiting histone deacetylases and REST at neuron-restrictive silencer elements. Represses the CLOCK-BMAL1 heterodimer-mediated transcriptional activation of the core clock component PER2 (By similarity).
