A cellular constituent that is one of the building blocks of ribonucleic acids (RNA) and deoxyribonucleic acid (DNA). In biological systems, nucleotides are linked by enzymes in order to make long, chainlike polynucleotides of defined sequence. The order or sequence of the nucleotide units along a polynucleotide chain plays an important role in the storage and transfer of genetic information. Many nucleotides also perform other important functions in biological systems. Some, such as adenosine triphosphate (ATP), serve as energy sources that are used to fuel important biological reactions. Others, such as nicotinamide adenine dinucleotide (NAD) and coenzyme A (CoA), are important cofactors that are needed to complete a variety of enzymatic reactions. Cyclic nucleotides such as cyclic adenosine monophosphate (cAMP) are often used to regulate complex metabolic systems. Chemically modified nucleotides such as fluoro-deoxyridine monophosphate (Fl-dUMP) contain special chemical groups that are useful for inactivating the normal function of important enzymes. These and other such compounds are widely used as drugs and therapeutic agents to treat cancer and a variety of other serious illnesses.