It occurs in combined form in nucleic acids and several coenzymes. In DNA its complementary Base is guanine. It or its corresponding nucleoside or nucleotide may be prepared from DNA by selective techniques of hydrolysis.

• 在核酸和几种辅中以结合的形式出现。在去氧核糖核酸(DNA)中,其互补基是鸟嘌呤。借着水解的挑选技术,可从去氧核糖核酸制备胞嘧啶或相关的核或核酸。