What determines the behavior of a cell, for example whether a cell will divide or not, whether a cell will fulfill a function, or whether a cell will undergo programmed cell death or instead acquire features reminiscent of immortality (a hallmark of cancer), is entirely determined by the concerted activities of a cell’s genes. The activity of any given gene is primarily determined by DNA sequences that are juxtaposed to the gene, such as those that make up a functional unit named the promoter, which as the name indicates, promotes the associated gene’s activity. The promoter enables a gene to be active by binding to proteins that either facilitate or drive the gene’s activity. Thus, to be fully functional, a gene’s promoter has to be accessible to the proteins that need to bind to it. While genetic abnormalities that change the DNA sequence of a gene or of its promoter such as those that result from mutations, deletions, or translocations (whereby two chromosomal fragments are fused together) can change a gene’s activity, we are discovering that there are several phenomena that can affect a gene’s activity, independently of the actual DNA sequence. Thus, in many of these instances, while the DNA sequence is intact and presents no abnormalities whatsoever, the chemistry that is superimposed to a gene’s DNA sequence, or that of the chromosomal components where the gene resides, can be changed to the extent that a gene’s promoter is no longer accessible to its binding proteins, or instead, accessible all or most of the time as opposed to the normal state which is only during certain periods of the cell’s life cycle. In other instances a gene is active, but before it can translate into a product (a protein) that regulates cellular functions, it is prevented from proceeding to this step by the actions of abnormally expressed RNA molecules such as miRNAs (microRNAs) or lncRNA (long non-coding RNAs). Together, the processes that affect a gene’s activity without affected its DNA sequence constitute the epigenetics of a cell.