In every cell, DNA is tagged with chemical groups-like flags on the DNA-that make up the epigenetic code and act as guideposts. These flags help the cell's machinery know when to activate or inactivate a gene. In healthy cells, these flags are correctly placed and genes are expressed in the right proportions. However, in many tumor cells the flags are incorrectly applied. One form of incorrect DNA flagging occurs when extreme amounts of otherwise healthy methyl chemical groups, which act like on/off switches for a gene, are incorrectly added to many places on the cell's DNA. Instead of a cell chugging along with its natural, biological jobs, these newly added methyl groups cause the cell to act erratically, leading to the cell expressing genes at the wrong levels, or not at all. This generally disrupts the normal flow of information in the cell. Importantly, these flags can turn off key defensive genes that help protect the cell from becoming cancerous and disrupt other processes. When this occurs, it's called "CpG Island Methylation Phenotype," or CIMP for short.
Credit: Darryl Leja, NHGRI.