PHF8 (UniProtKB/Swiss-Prot entry Q9UPP1) is a jumonji domain containing protein which plays role in histone demethylation. PHF8 selectively demethylates the di- and monomethyl H3K9, H3K27 and H4K20 and acts as a transcriptional activator. It plays a key role cell cycle progression, rDNA transcription and brain development. Mutations in PHF8 lead to Siderius type X-linked mental retardation (MRXSSD), a mild to borderline type of mental retardation.