Genomic imprinting – epigenetic modifications that ensure that certain genes are expressed from only one of the two inherited chromosomes – is crucial for normal development. Mammalian imprinted genes are associated with differentially methylated regions (DMRs) that are CpG methylated on one parental chromosome. At least 21 DMRs become methylated in the mouse germline and, on p. 811, Hiroyuki Sasaki and co-workers analyse a panel of these gametic DMRs. The extent of methylation of these DMRs differs significantly from that of embryonic DMRs, they report, suggesting that gametic DMRs should be used to identify the features that establish imprinting in the germline. They also show that maternal gametic DMRs appear as unmethylated islands in male germ cells, and unexpectedly identify widespread oocyte-specific non-CpG methylation. Finally, they report that DMR methylation changes dynamically during early development, indicating that DMRs are not fully protected from preimplantation epigenetic reprogramming. These results underscore the importance of using gametic DMR sequences for the study of imprint establishment.