There are two main forms of methylation – electrophilic methylation and nucleophilic methylation. 

1. In electrophilic methylations, electrophiles are the source of the methyl group that gets added to the lysine residues on the N-terminal tail of histone proteins. Diazomethane, iodomethane, dimethyl sulfate and dimethyl carbonate are common examples of electrophilic methylating agents. 
2. Nucleophilic methylationsoccur when a nucleophilic agent is used for the transfer of a methyl group. Tetramethyltin, dimethyl zinc, methyllithium, and trimethyl aluminum are common examples of nucleophilic methylating agents.