Epigenetics refers to non-coding sequence changes such as DNA methylation, histone modifications, chromosome remodeling and non-coding RNA regulation. Histone modifications include acetylation, phosphorylation, methylation, ubiquitination and ADP ribosylation. The combinations of different histone modifications, known as "histone code", are dynamic during development and differentiation and play important roles in the regulation of gene expressions in spatial-temporal manners. The modification on a particular residue in a histone affects not only the modifications at different residues in its own protein but also other histones. The histone modifications is a complicated network and the regulation remains elusive.