The M13 bacteriophage is a rod-shaped virus which infects bacteria. This virus contains a single stranded DNA and three different types of coat proteins. 2700 copies of the major coat protein are arranged in a repeating helical array, forming a flexible cylinder in which the DNA is encased. Five copies of the two minor coat proteins are situated at each end of the virus. The wild-type virus is approximately 860 nm long and 6.5 nm wide. The viral coat proteins can be easily modified by genetic engineering to insert nucleation centers on the virus surface or to specifically functionalize viruses with different binding motifs.