RNA oncovirus expression in N‐methyl‐N‐nitrosourea‐induced lymphomas in mice

RNA oncovirus expression was investigated in thymic lymphomas induced by N-methyl-N-nitrosourea (MNU) in (C57BL/6 × DBA/2)F1(BDF1) mice. Out of 15 newborn mice injected with 20 mg/kg MNU, five developed thymic tumours of T-cell origin. Virus production of thymic cells from tumorous and normal animals was studied by an infectious center assay. All five tumours released ecotropic (mouse tropic) virus. No xenotropic or dualtropic mink cell focus-inducing virus was found. No virus was obtained from thymus cells taken from six normal mice killed between 83 and 124 days; however, an ecotropic virus was cultured from the thymus of a 134-dayold normal mouse. According to these results, the MNU-induced thymic lymphoma development in BDF1, mice is accompanied by activation of endogenous ecotropic virus, whereas spontaneous activation also occurs in older mice.