Structural dissimilarity in insulin-like growth factor II genes of the rat and the human

A comparison of insulin-like growth factor II(IGFII) genes revealed the lack in the rat genome of a transcriptional system identified in human IGFII gene, h1-h2-h3. Complete deletion was, however, restricted to the h1 region, while the h2 and h3 sequences were conserved in a nearly intact form and in a highly deviated one, respectively. Meanwhile, the human genome contained a region highly homologous to the rE1 sequence, a rat-specific leader exon. These results indicate that the IGFII genes of these species were derived from a common ancestral gene having, together with two other common leader exons, rE2 (h4) and rE3 (h4B), four leader exons, and that the human-specific leader exon h1 sequence was not newly acquired in the human genome but was deleted from the rat genome after speciation.