Epigenetics is the study of changes in gene expression that occur without altering the DNA sequence. The Greek prefix epi- in epigenetics implies features that are "on top of" or "in addition to" the traditional DNA-sequence-based mechanism of inheritance. Epigenetics usually involves changes that persist through cell division, and affect the regulation of gene expression. Such effects on cellular and physiological traits may result from environmental factors, or be part of normal development.