Cells from individuals with Major Depressive Disorder (MDD) were found to have higher than expected rates of methylation at specific sites on their DNA, when compared to cells from healthy individuals without MDD, according to a study by a multidisciplinary team of UC San Francisco scientists, in collaboration with others. Methylation is a process by which DNA is chemically modified at specific sites, resulting in changes in the expression of certain genes. Methylation of particular sets of genes, called “DNA methylation clocks,” typically change in predictable ways as people age, but the rate of these changes varies between people. Methylation patterns in individuals with MDD suggested that their cellular age was, on average, accelerated relative to matched healthy controls.