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. It plays a crucial role in regulating various cellular functions, including development, differentiation, and response to environmental factors. Methylation patterns can vary between different cell types and can be inherited or acquired throughout an organism's life. Abnormal DNA methylation is associated with various diseases, including cancer, where it can lead to the silencing of tumor-suppressor genes or the activation of oncogenes.