DNA and RNA have four building blocks each (the nucleosides or nucleotides). There are more than 100 other naturally occurring modified nucleosides. Some As-containing nucleosides (sugar derivatives) are also known. Purines are synthesized as nucleosides (bases attached to ribose). Lamivudine has reverse chirality compared to didanosine, stavudine, zidovudine, and natural nucleosides. Nucleoside phosphoramidites are derivatives of natural or synthetic nucleosides. Thus, an intracellular (mono)nucleotide delivery should have maximal impact for d4T and similar nucleosides. The remaining three nucleosides may be found in both RNA and DNA. Examples include nucleosides such as adenosine. It can also come with lipoic acid and nucleosides. Purines are biologically synthesized as nucleosides (bases attached to ribose). Antiviral drugs are often nucleoside analogues, which masquerade as DNA building blocks (nucleosides). Nicotinamide adenine dinucleotide consists of two nucleosides joined by a pair of bridging phosphate groups. Transfer RNA (tRNA) molecules contain a particularly large number of modified nucleosides. Editing occurs post transcriptionally as the nascent polynucleotides do not contain edited nucleosides.