Bacteriophage MS2 (Emesvirus zinderi), commonly called MS2, is an icosahedral, positive-sense single-stranded RNA virus that infects the bacterium Escherichia coli and other members of the Enterobacteriaceae.

The virus was isolated in 1961 and its genome was the first to be fully sequenced, in 1976, providing a crucial understanding of genetic codes.

The virus is also under research for potential uses in drug delivery, tumor imaging, and light harvesting.

[7] MS2 infects enteric bacteria carrying the fertility (F) factor, a plasmid that allows cells to serve as DNA donors in bacterial conjugation.

[8] A LS dipeptide motif on the L protein is found throughout the genus Levivirus and appears to be essential to the lysis activity, although their different locations suggest that they have evolved independently.

[9] In 1961, MS2 was isolated by Alvin John Clark and recognized as an RNA-containing phage very similar to bacteriophage f2.

[3] This was accomplished by Walter Fiers and his team, building upon their earlier milestone in 1972 of the first gene to be completely sequenced, the MS2 coat protein.

[13] Since 1998,[14] the MS2 operator hairpin and coat protein have found utility in the detection of RNA in living cells (see MS2 tagging).

MS2 and other viral capsids are also currently under investigation as agents in drug delivery, tumor imaging, and light harvesting applications.