The function of KDM5B (lysine demethylase 5B, Ensembl gene identifier ENSG00000117139) is as follows. Histone demethylase that demethylates 'Lys-4' of histone H3, thereby playing a central role in histone code (PubMed:24952722, PubMed:27214403, PubMed:28262558). Does not demethylate histone H3 'Lys-9' or H3 'Lys-27'. Demethylates trimethylated, dimethylated and monomethylated H3 'Lys-4'. Acts as a transcriptional corepressor for FOXG1B and PAX9. Favors the proliferation of breast cancer cells by repressing tumor suppressor genes such as BRCA1 and HOXA5 (PubMed:24952722). In contrast, may act as a tumor suppressor for melanoma. Represses the CLOCK-BMAL1 heterodimer-mediated transcriptional activation of the core clock component PER2 (By similarity). {ECO:0000250|UniProtKB:Q80Y84, ECO:0000269|PubMed:12657635, ECO:0000269|PubMed:16645588, ECO:0000269|PubMed:17320161, ECO:0000269|PubMed:17363312, ECO:0000269|PubMed:24952722, ECO:0000269|PubMed:26645689, ECO:0000269|PubMed:26741168, ECO:0000269|PubMed:27214403, ECO:0000269|PubMed:28262558}.