DNA methylation is a biological process where a methyl group is added to the DNA molecule, typically at cytosine bases. This modification can influence gene expression without changing the underlying DNA sequence. Methylation often occurs in regions of DNA known as promoters, which control the activity of genes. This process plays a crucial role in various cellular functions, including development, cell differentiation, and genomic stability. Abnormal DNA methylation patterns are associated with several diseases, including cancer, where genes that normally suppress tumors may become silenced, leading to uncontrolled cell growth.