Fig. 1. Genomic organisation and conservation of HUC1 and HUC2 sequences in mouse and human. (A) Mouse H19/Igf2 imprinted locus. H19 is expressed only from the maternal allele, Igf2 only from the paternal allele and Rpl23 is bi-allelically expressed. Characterised enhancers downstream of H19 (green circles) mediate expression in both endodermal and some mesodermal tissues (Leighton et al., 1995; Brenton et al., 1999; Ishihara et al., 2000). The ICR at H19 (red ellipse) and the silencer located within DMR1 upstream of Igf2 (blue diamond) (Constancia et al., 2000) are also shown. (B) The conserved structural organisation of sequences at the H19 gene. The extended sequenced region upstream of H19 (AF327412) is indicated. The top line represents mouse DNA sequence, aligned with human sequence on the bottom line (PAC clone pDJ998n23, EM:AC004556) using BlastN software. The numbers above and below the lines relate to the 5' boundary of various regions relative to the H19 gene transcriptional start site. Percentages shown in between the two sequences are the degree of homology at the nucleotide level and the approximate length in kb of the conserved region is shown in brackets. The H19 transcribed region (white box) shows 92% homology between the two species. This degree of conservation is almost matched by HUC1 and HUC2 (yellow boxes), which have a greater level of sequence homology than the characterised downstream enhancer elements (green circles) (Leighton et al., 1995). Conversely, sequence analysis does not reveal a human homologue of the ICR, which comprises both silencer and boundary functions (red ellipse), located upstream of mouse H19. (C) Dotplot sequence comparison of human and mouse H19 upstream region. GCG dotplot software only identified the H19 transcribed sequence and HUC1 and HUC2 regions, as conserved between mouse and human (default parameters) in the upstream region.