Polycomb group (PcG) protein. Catalytic subunit of the PRC2/EED-EZH1 complex, which methylates 'Lys-27' of histone H3, leading to transcriptional repression of the affected target gene. Able to mono-, di- and trimethylate 'Lys-27' of histone H3 to form H3K27me1, H3K27me2 and H3K27me3, respectively. Required for embryonic stem cell derivation and self-renewal, suggesting that it is involved in safeguarding embryonic stem cell identity. Compared to EZH2-containing complexes, it is less abundant in embryonic stem cells, has weak methyltransferase activity and plays a less critical role in forming H3K27me3, which is required for embryonic stem cell identity and proper differentiation. L-lysyl(27)-[histone H3] + 3 S-adenosyl-L-methionine = 3 H(+) + N(6),N(6),N(6)-trimethyl-L-lysyl(27)-[histone H3] + 3 S-adenosyl-L-homocysteine Component of the PRC2/EED-EZH1 complex, which includes EED, EZH1, SUZ12, RBBP4 and AEBP2. The PRC2/EED-EZH1 is less abundant than the PRC2/EED-EZH2 complex, has weak methyltransferase activity and compacts chromatin in the absence of the methyltransferase cofactor S-adenosyl-L-methionine (SAM). Interacts with EZHIP; the interaction blocks EZH1 methyltransferase activity. Colocalizes with trimethylated 'Lys-27' of histone H3. Belongs to the class V-like SAM-binding methyltransferase superfamily. Histone-lysine methyltransferase family. EZ subfamily.