Imprinted genes play important roles in the regulation of growth and 
development, and several have been shown to influence behavior. Their 
allele-specific expression depends on inheritance from either the mother or the 
father, and is regulated by "imprinting control regions" (ICRs). ICRs are 
controlled by DNA methylation, which is present on one of the two parental 
alleles only. These allelic methylation marks are established in either the 
female or the male germline, following the erasure of preexisting DNA 
methylation in the primordial germ cells. After fertilization, the allelic DNA 
methylation at ICRs is maintained in all somatic cells of the developing embryo. 
This epigenetic "life cycle" of imprinting (germline erasure, germline 
establishment, and somatic maintenance) can be disrupted in several human 
diseases, including Beckwith-Wiedemann syndrome (BWS), Prader-Willi syndrome 
(PWS), Angelman syndrome and Hydatidiform mole. In the neurodevelopmental Rett 
syndrome, the way the ICR mediates imprinted expression is perturbed. Recent 
studies indicate that assisted reproduction technologies (ART) can sometimes 
affect the epigenetic cycle of imprinting as well, and that this gives rise to 
imprinting disease syndromes. This finding warrants careful monitoring of the 
epigenetic effects, and absolute risks, of currently used and novel reproduction 
technologies.