Fig. 4. Methylation status of individual DNA strands in (A) the H19 upstream DMD and (B) Snrpn promoter-exon 1 region in embryos and placentas recovered at embryonic day 9.5 as determined by bisulfite analysis. Details are as described in Fig. 2. A low level of sporadic methylation on the normally unmethylated allele was observed in samples perturbed by preimplantation culture (data not shown).