**1. Introduction**

320 Breast Cancer – Focusing Tumor Microenvironment, Stem Cells and Metastasis

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In eukaryotic cells, DNA is maintained in a highly ordered and condensed form via its association with small, basic histone proteins. The fundamental subunit of chromatin, the nucleosome, is composed of an octamer of four core histones, an H3/H4 tetramer and two H2A/H2B dimers, around which 146 bp of DNA are wrapped. Dynamic modulation of chromatin structure, that is, chromatin remodeling, is a key component in the regulation of gene expression, apoptosis, DNA replication and repair and chromosome condensation and segregation. Enzymes that eovalently modify histones control many cellular processes by affecting gene expression. These modifications of core histones mainly include of methylation, acetylation, phosphorylation, ubiquitination/sumoylation, ADPribosylation, deamination, and proline isomerisation (Ito, 2007; Bartova et al., 2008). The abnormal regulation of these processes is intimately associated with human diseases, including cancer.

Breast cancer, the leading cause of death from cancer in women, is a heterogeneous disease ranging from premalignant hyperproliferation to invasive and metastatic carcinomas (Jemal et al., 2011). The disease progression is poorly understood but is likely due to the accumulation of genetic mutations leading to widespread changes in gene expression. Accumulating evidence has suggested that abnormal alteration of histone modification plays roles in the process of breast cancer. This chapter will summarize the relationship between histone modification and the molecular mechanism of breast cancer, and the therapy strategies focused on histone modification for breast cancer will also be discussed.
