assemble into an octomer that DNA wraps around to create a unit called a nucleosome. The ability of a histone to be modified by being marked on its N-terminal tail is essential for modulating gene expression. In its most basic sense, histones can be modified to become more or less condensed by histone deacetylases (HDACs) or histone acetyltranferases (HATs), respectively. When histones are acetylated and become less condensed as a result, it allows for the underlying DNA to be more accessible to be transcribed, increasing gene expression. Thus, acetylated histones can serve as binding sites for important transcription initiation factors and enzymes that have the ability to remodel chromatin. Meanwhile, increased condensation of histones via deacetylation results in the opposite effect, causing decreased gene expression. The regulation of gene expression can have implications on which genes are transcribed, ultimately affecting protein production in the human body. Though histone tails can be marked in many different ways, four prominent modifications will be discussed in this article: acetylation, methylation (of histones), ubiquination, and phosphorylation. In addition, methylated histones can both serve as binding sites for certain transcription factors due to their bromodomains and chromodomains, and also prevent the binding of such factors by hiding the recognition site of the transcription factor. Transcription factors, including promoters and enhancers, can subsequently modulate the rate at which DNA is transcribed to RNA, also affecting gene expression. Effects on cancer Physical exercise leads to epigenetic modifications that can have beneficial effects in cancer patients. The effect of physical exercise on DNA methylation patterns leads to increased expression of genes associated with tumor suppression and decreased expression of oncogenes. Cancer cells have non-normal patterns of DNA methylation including hypermethylation in promoter regions for tumor-suppressing genes and hypomethylation in promoter regions of oncogenes. These epigenetic mutations in cancer cells cause the cell to grow and divide uncontrollably, resulting in tumorigenesis. Physical exercise has been shown to reduce and even reverse these epigenetic mutations, increasing expression levels of tumor-suppressing genes and decreasing expression levels of oncogenes. Hypermethylation in the promoter regions of tumor suppressor genes is thought to help cause some forms of cancer. The hypermethylation in the promoter regions of the tumor suppressing genes APC and RASSF1A are common epigenetic markers for cancer. The APC gene functions to make sure cells divide properly and maintain a correct number of chromosomes after division has completed. The RASSF1A gene product