The imprinted expression of the mouse Igf2/H19 locus is governed by the 
differential methylation of the imprinting control region (ICR), which is 
established initially in germ cells and subsequently maintained in somatic 
cells, depending on its parental origin. By grafting a 2.9-kbp H19 ICR fragment 
into a human beta-globin yeast artificial chromosome in transgenic mice, we 
previously showed that the ICR could recapitulate imprinted methylation and 
expression at a heterologous locus, suggesting that the H19 ICR in the 
beta-globin locus contained sufficient information to maintain the methylation 
mark (K. Tanimoto, M. Shimotsuma, H. Matsuzaki, A. Omori, J. Bungert, J. D. 
Engel, and A. Fukamizu, Proc. Natl. Acad. Sci. USA 102:10250-10255, 2005). 
Curiously, however, the transgenic H19 ICR was not methylated in sperm, which 
was distinct from that seen in the endogenous locus. Here, we reevaluated the 
ability of the H19 ICR to mark the parental origin using more rigid criteria. In 
the testis, the methylation levels of the solitary 2.9-kbp transgenic ICR 
fragment varied significantly between six transgenic mouse lines. However, in 
somatic cells, the paternally inherited ICR fragment exhibited consistently 
higher methylation levels at five out of six randomly integrated sites in the 
mouse genome. These results clearly demonstrated that the H19 ICR could acquire 
parent-of-origin-dependent methylation after fertilization independently of the 
chromosomal integration site or the prerequisite methylation acquisition in male 
germ cells.