**2.2.2.2 Histone arginine methyltransferase (HRMTs)**

The protein arginine methyltransferase (PRMT) family is the major HRMTs up to now. The PRMTs are classified into four groups depending on the type of methylarginine they generate: Type I PRMTs (PRMT1, PRMT2, PRMT3, PRMT4, PRMT6 and PRMT8) catalyze the formation of ω-NG, monomethylarginines (MMA) and ω-NG, NG-asymmetric dimethylarginines (aDMA); Type II PRMTs (PRMT5, PRMT7 and PRMT9) catalyze the formation of MMA and ω-NG, N'G-symmetric dimethylarginines (sDMA); Type III PRMTs (remained unclear) catalyze only the monomethylation of arginine residues in proteins; Type IV PRMTs (only be found in *Saccharomyces cerevisiae* up to date) catalyze the methylation at delta (Δ) nitrogen atom of arginine residues (Niewmierzycka et al., 1999; Boisvert et al., 2005; Bachand, 2007).

Compared to HKMTs, The evidence for the involvement of HRMTs in human cancers is not as solid. However, underexpression of PRMT1 has been observed in breast cancer (Scorilas et al., 2000). PRMT4, also known as coactivator-associated arginine methyltransferase-1 (CARM1), is a coactivator for nuclear receptors and is oversexpressed in prostate and breast cancers (El et al., 2006). PRMT4 plays an important role in estrogeninduced cell cycle progression in the MCF-7 breast cancer cell line. Upon estrogen stimulation, the E2F1 promoter is subject to PRMT4-dependent dimethylation on H3R17, and this recruitment of PRMT4 by ERα are dependent on the presence of the NCOA3 (Frietze et al., 2008).
