CpG methylation may occur rapidly and may persist for decades. For instance, when rats were exposed to fear conditioning, methylation of the protein phosphatase 1 gene pp1 was increased and methylation of the reelin gene reln was decreased in 1 h (Miller and Sweatt 2007). The methylation of these genes returned to baseline within 24 h. Methylation status of the promoter of the imprinted insulin-like growth factor 2 IGF2 gene, a gene involved in energy metabolism, was examined in subjects prenatally exposed to famine during the Dutch Hunger Winter of 1944–1945 (Heijmans et al. 2008). It was found that the promoter of the IGF2 gene was hypomethylated compared with controls, presumably reflecting events that occurred 60 years ago.
