and Stockert, 1977). The isolation and characterization of MuLVs require specialized techniques usually available only in research laboratories dedicated to viral oncology. However, MuLV type and group specificity can be determined by immunofluorescence of fixed cells by use of an appropriate panel of type- or group-specific antisera (Matthews, 1982).
All mice probably have vertically transmitted endogenous MuLV proviruses as an integral part of their genomes; this is true even for mice that are otherwise considered germfree (Kajima and Pollard, 1965). Horizontal transmission can occur but is considered inefficient (Furmanski and Rich, 1982). Therefore, control measures are not usually considered useful unless mice are being infected experimentally with high doses of MuLV, in which case the infected mice should be isolated from uninfected control mice.
Although all mice have endogenous MuLVs, their presence probably has little significance for most research purposes in which mice are used. MuLV expression and the associated occurrence of neoplasms, however, can present competing endpoints in some studies, e.g., studies of the aging processes in various organs. Thus, an awareness of spontaneous tumors in different strains of mice or the induction of tumors by specific test chemicals can be useful in the design of some experiments. Also, active MuLV infection can cause suppression of humoral and cellular immunity without clinically obvious disease (Specter et al., 1978).