The locus was replaced with an H19Xba cassette consisting of murine H19 sequence from 4 kb upstream of the transriptional start site to 7.5 kb downstream of the gene which has been modified with the inserion of 2 XbaI linkers into a SacI site at +580 bp in exon 1. The knockin mutation conserves all sequences at the endogenous locus, with the placement of a single loxP site upstream of the gene and a neo-loxP cassette downstream of the gene. Northern blot analysis indicates expression of H19 RNA is reduced to 57% of wild-type in heterozygotes with a maternally inherited mutant allele.