We have identified a region with characteristics of a paternal-specific 
methylation imprint at the human H19 locus. This region, extending from -2.0 kb 
upstream to the start of transcription, is heavily methylated in sperm and on 
the paternal allele in somatic cells. This methylation was preserved during 
pre-implantation. Structural analysis revealed the presence of CpG islands and a 
large direct repeat with a 400 bp sequence reiterated several times, but no 
significant sequence homology to the corresponding region of the mouse H19 gene. 
These findings could suggest a role for secondary DNA structure in genomic 
imprinting across the species, and they also present a puzzling aspect of the 
evolution of the H19 regulatory region in human and mouse.