Cytosine is a base component of nucleic acids that can be found in deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). In the double-stranded helical strand of DNA, cytosine on one single strand, can pair with guanine on the other single strand and three hydrogen bonds are formed between molecules, thus maintaining the stability of the DNA double helix structure. In addition, cytosine and cytosine nucleotides, are used as drugs for the synthesis of ascending leukocytes. Clinically used antiviral drugs, such as anti-HIV and anti-hepatitis B virus drugs, lamivudine, and anti-cancer drugs, gemcitabine, enanthate, and the commonly used 5-fluorocytosine, among others, are synthesized from cytosine.