Xanthosine is a nucleoside derived from xanthine and ribose.

It is the biosynthetic precursor to 7-methylxanthosine by the action of 7-methylxanthosine synthase.

7-Methylxanthosine in turn is the precursor to theobromine (active alkaloid in chocolate), which in turn is the precursor to caffeine, the alkaloid in coffee and tea.

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