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The structure of the 5' methylated cap of eukaryotic mRNA. The distinguishing chemical features are the 5'-to-5' linkage of 7-methylguanylate to the initial nucleotide of the mRNA molecule and the methyl group at the 2' hydroxyl of the ribose of the first nucleotide (Base 1). Both these features occur in all animal cells and in cells of higher plants; yeast lack the methyl group on Base 1. The ribose of the second nucleotide (Base 2) also is methylated in vertebrates.