Ribothymidine is an endogenous methylated nucleoside found in human fluids; methylated purine bases are present in higher amounts in tumor-bearing patients compared to healthy controls.

Methyl nomilinate from citrus can modulate cell cycle regulators to induce cytotoxicity in human colon cancer (SW480) cells in vitro.

Xanthosine is produced by guanine-free mutants of bacteria e. g. Bacillus subtilis, Aerobacter aerogenes.