Strain of origin - this allele was found in C57BL/6, C58/J, C57BR, MA/My strains

This locus controls resistance and susceptibility of cells in tissue culture to infection by mink cell focus-forming murine leukemia viruses. The allele Rmcf^r determines resistance and occurs in strains DBA/1, DBA/2, and CBA/Ca; the allele Rmcf^s determines susceptibility and occurs in strains AKR/J, C57BL/6, BALB/c, CBA/J, NFS, NZB, 129/J, and many others. Heterozygotes are as resistant as the resistant parent or nearly so. Rmcf is different from and independent of Fv1, a locus that controls susceptibility to infection by ecotropic viruses. Rmcf is located on Chr 5 close to Hm near the centromeric end (J:7108). Rmcf^r protects (AKR x CBA/Ca)F1 and (AKR x DBA/2)F1 hybrids from development of spontaneous thymic lymphomas and reduces the incidence of MCF-induced thymic lymphomas (J:7175). It also reduces susceptibility of cells of Sxv^s/Sxv^r mice to exogenous xenotropic viruses (J:7951). In addition, in strains susceptible to Friend virus-induced erythroleukemia, a condition thought to be due to the replication of MCF virus, Rmcf^r increases resistance to the virus-induced erythroleukemia. It may cause resistance by coding for or regulating the production of an MCF-related envelope glycoprotein that blocks the receptor for MCF viruses (J:8074). This conclusion is reinforced by the findings of Buller et al. (J:8497), who showed that the Rmcf^r allele contains an endogenous MCF gp70 env gene and that the Rmcf^s allele, at least in some strains (C57BL/6, CBA/J, and A/WySn), contains a xenotropic gp70 env gene. Presumably the MCF gp70 inhibits exogenous MCF infection in vitro by a mechanism of viral interference.

The Mx genes determine resistance to the lethal effects of various myxoviruses including neurotropic avian influenza A virus injected intracerebrally, pneumotropic strains injected intranasally, and a hepatotropic strain injected intraperitoneally (J:5645, J:13136). Resistance is not dependent on presence of the thymus and is not abolished by immunosuppression or by inhibitors of macrophage function (J:5735, J:5478, J:5645). Resistance is specific for the orthomyxoviruses (J:6265). It is dependent on the presence of interferon-alpha and -beta but not -gamma (J:7365).

The resistance allele at the Mx1 locus, under induction by alpha/beta interferon, produces the 75 kDa protein MX-1, which confers resistance to the influenza virus, in the nuclei of cells carrying the allele. Susceptibility alleles do not produce the protein (J:8273). The protein is located in the nucleus (J:7703) and produces its antiviral effect by preventing synthesis of viral mRNA in the nucleus (J:7992). Nuclear localization is necessary for anti-influenza virus activity (J:1417), but mutations induced in Mx1 showed that nuclear position was not sufficient for the effect; mutations in several domains can cause its loss (J:11840). The MX-1 protein is a GTPase containing a GTP binding domain (J:1417) and this binding core is also necessary (J:21243).

Resistance is expressed by macrophages and other cells in vitro (J:6649, J:5940) but could not be transferred to susceptible animals by transfer of macrophages from resistant mice (J:6149).

Resistance to infection with two tick-borne viruses, Thogoto virus (J:8273) and Dhori virus (J:27760), is also conferred by Mx1^r.

The Mx1^r allele occurs only in strains A2G, SL/NiA, and T9, the latter being a strain derived from an influenza-resistant wild stock, and CAST/Ei, derived from Mus musculus castaneus. Most inbred strains, including C57BL/6J, C3H/HeJ, and BALB/cJ, carry an influenza susceptible Mx1^s1 allele which produces mRNA lacking exons 9, 10, and 11 of the Mx1^r allele. This large deletion apparently renders the protein incapable of providing resistance to influenza. The CBA/J, CE/J, I/LnJ, and PERA/Ei strains, also susceptible to the virus, have another form of the Mx1^s2 allele in which there is a nonsense mutation (J:9452).

Interferon is induced by viral infection and in turn induces the Mx protein (J:7703). Although some interferon-induced genes respond directly to virus invasion as well as indirectly through induction by virus-induced interferon, this primary response is very weak for the MX-1 protein in response to either influenza or Newcastle disease viruses (J:1892).