de novo methylated in both groups. The 5' and 3' CpG sites of the exon 17 GR NGFIA response element in the offspring of both high- and low-LG mothers, which exhibit differential methylation later in life, are de novo methylated to the same extent. These data show that both the basal state of methylation and the first wave of de novo methylation after birth occur similarly in both groups. Whereas it is generally accepted that DNA methylation patterns are formed prenatally and that de novo methylation occurs early in development, there is at least one documented example of postnatal de novo methylation of the HoxA5 and HoxB5 genes.120 Since similar analyses are not documented for other genes, it is unknown yet whether changes in methylation are common around birth or whether they are unique to this GR promoter. One aspect of these findings that is important is that of the complete absence of cytosine methylation on embryonic day 20. Since the majority of the pyramidal cells of Amnion's Horn are born between embryonic days 16 and 20, it seems unlikely that methylation patterns, at least on the exon 17 promoter of the GR, are generated at the time of DNA
