6 variations of dinucleotides (DNA has 4 bases thus 16 variations) not all of the methylation occurs in these CpG dinucleotides. The frequency with which the CpG dinucleotide recurs in the human genome should be about 6%, but in actuality, it's only about 5-10% of it's predicted frequency (Das and Singal, 2004). It is predicted that the suppression of CpG directly corresponds to the abnormally frequent mutation of the methylated cytosine. This is direct evidence that methylation is not consistently distributed throughout the human genome- unmethylated regions occur irregularly and unevenly throughout the methylated regions. An example of this would be the CpG islands dispersed throughout the genome that are mostly unmethylated and found in promotors of about 60% of genes (Kondo and Jean-Pierre, 2010). These regions contain a high density of GC nucleotides thus deviating from the conventional suppression of CpG dinucleotides. Similar to the mechanism of silencing of CpG dinucleotides (due to methylation), methylation in CpG islands in the promoter region directly correlates to gene silencing, more specifically, tumor-suppressor gene inactivation. A more in depth analysis of tumerogenesis has revealed that cancer cells, more often than not, are subject to overall hypomethylation and specific areas of hypermethylation at particular gene-associated CpG islands concurrently.
             Prior to the mid 1980s, only hypomethylation of DNA was associated with cancer, thus served to be the cornerstone of tumor research
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