Genus Betaretrovirus

Type Species	Mouse mammary tumor virus	(MMTV)

Distinguishing Features

Virions of Mouse mammary tumor virus (MMTV) exhibit a ‘B-type’ morphology with prominent surface spikes and an eccentric condensed core. Other members of the Genus have a ‘D-type’ morphology with less dense surface spikes and a cylindrical core. Capsid assembly occurs within the cytoplasm (to yield structures previously termed ‘A-type’ particles) prior to transport to, and budding from the plasma membrane. Protein Mr are: MA   ± 10 10³; p21 21 10³; p8/p12 8-12 10³; CA   ± 27 10³; NC   ± 14 10³; DU; PR   ± 15 10³; RT; IN; SU   ± 52 10³; TM   ± 36 10³. The genome is 8-10  kb in size (one monomer); its organization for MMTV is illustrated in Figure 3.

In MMTV there is an additional gene (sag) whose product functions as a superantigen and is located at the 3-end of the genome, overlapping U3. This gene is absent in other members of the genus. The tRNA primer is tRNA^Lys; tRNA^Lys-3 for MMTV and tRNA^Lys-1,2 for other members of the genus. The LTR of MMTV is about 1300  nts long primarily due to the sag encoding U3 region of 1200  nts. The R sequence (15  nts) and the U5 region (95-120  nts) are of similar length in all members of the genus.

Viruses assigned to this genus include exogenous, vertically-transmitted (milk) and endogenous viruses of mice, as well as exogenous, horizontally transmitted and endogenous viruses of new and old world primates and sheep. Murine viruses are associated with mammary carcinoma and T-lymphomas, while the exogenous primate viruses are associated with immuno-deficiency diseases; Jaagsiekte sheep virus is associated with pulmonary cancer of sheep. No oncogene-containing member is known.

List of Species Demarcation Criteria in the Genus

The list of species demarcation criteria is:

	Differences in genome sequence,
	Differences in gene product sequences,
	Differences in natural host range,
	Different oncogenes that may be incorporated.

Several primate retroviruses have been described that appear to be divergent members of a single virus that arose from a recombination event in which the env gene of a primate Gammaretrovirus was captured. The most divergent of these are the endogenous Squirrel monkey retrovirus (SMRV) and Langur virus (LNGV), which are unable to infect cells from the primate species of origin. Several serologically distinct strains exist within the Mason-Pfizer monkey virus (M-PMV) species. The most divergent of these are the endogenous SMRV and LNGV, nevertheless, the most closely related species Mason-Pfizer monkey virus (MPMV), Simian retrovirus-1 (SRV-1) and Simian retrovirus-2 (SRV-2) are serologically distinct. Mouse mammary tumor virus (MMTV) is assigned to a separate species because of the unique sag coding region and a widely divergent and distinct env gene. Ovine pulmonary adenocarcinoma virus (JSRV) is also assigned to a separate species on the degree of nucleotide sequence divergence. Related endogenous proviruses have been identified in other mammalian species (rodents, primates).

List of Species in the Genus

Official virus species names are in italics. Tentative virus species names, alternative names ( ), strains or serotypes are not italicized. Virus names, genome sequence accession numbers [ ], and assigned abbreviations ( ) are:

Species in the Genus

Langur virus		(LNGV)
Mason-Pfizer monkey virus	[M12349]	(MPMV)
Simian retrovirus 1	[M11841]	(SRV-1)
Simian retrovirus 2	[M16605]	(SRV-2)
Mouse mammary tumor virus	[M15122]	(MMTV)
Ovine pulmonary adenocarcinoma virus (Jaagsiekte sheep retrovirus)	[M80216]	(JSRV)
Squirrel monkey retrovirus	[M23385]	(SMRV)

Tentative Species in the Genus

None reported.