Nucleic acids present in all living cells in combination with proteins to form conjugated protein called nucleoprotein. Nucleic acids are polymers of specific nucleotides. The nucleic acids are of two different types: deoxyribonucleic acid (DNA), ribonucleic acid (RNA). The combination of a base and sugar is called a nucleoside. Nucleotides also exist in activated forms containing two or three phosphates, called nucleotide diphosphates or triphosphates. If the sugar in a nucleotide is deoxyribose, the nucleotide is called a deoxynucleotide; if the sugar is ribose, the term ribonucleotide is used. The nitrogenous bases of nucleic acids are from purine and pyrimidine bases. The purine bases are adenine and guanine. Pyrimidine bases are thymine, cytosine and uracil. There are many purine and pyrimidine analogs used in the treatment of various cancers.