DNA methylation is a key epigenetic mechanism involved in the developmental 
regulation of gene expression. Alterations in DNA methylation are established 
contributors to inter-individual phenotypic variation and have been associated 
with disease susceptibility. The degree to which changes in loci-specific DNA 
methylation are under the influence of heritable and environmental factors is 
largely unknown. In this study, we quantitatively measured DNA methylation 
across the promoter regions of the dopamine receptor 4 gene (DRD4), the 
serotonin transporter gene (SLC6A4/SERT) and the X-linked monoamine oxidase A 
gene (MAOA) using DNA sampled at both ages 5 and 10 years in 46 MZ twin-pairs 
and 45 DZ twin-pairs (total n=182). Our data suggest that DNA methylation 
differences are apparent already in early childhood, even between genetically 
identical individuals, and that individual differences in methylation are not 
stable over time. Our longitudinal-developmental study suggests that 
environmental influences are important factors accounting for interindividual 
DNA methylation differences, and that these influences differ across the genome. 
The observation of dynamic changes in DNA methylation over time highlights the 
importance of longitudinal research designs for epigenetic research.