CAMBRIDGE, MA -- DNA is made from four nucleosides, each known by its own letter -- A, G, C, and T. However, since the structure of DNA was deciphered in 1953, scientists have discovered several other variants that are often added to the DNA sequences to replace one of the usual four letters.
These variants, which may be modified versions of the traditional nucleosides, often help cells to control which genes are turned on and off, and are referred to as "epigenetic marks" in the DNA. In bacteria, they can also protect DNA from invasion by other organisms such as viruses.