BALB/c, C3H, and AKR strains of mice were analyzed for the concentrations of the major internal protein of about 30,000 daltons and the major envelope glycoprotein of about 70,000 daltons of endogenous type-C oncornaviruses in embryos isolated during the second half of gestation, in newborn mice, and in specific organs of adult animals. Each strain showed a different pattern of protein expression. BALB/c tissues contained very low concentrations of both proteins (< 1 ng/mg of protein) in early embryos and adult tissues, whereas moderate levels of both were present in late embryos and newborn animals. C3H tissues contained high concentrations of glycoprotein throughout embryogenesis and in the newborn and the adult spleen; low levels of internal protein were also present throughout. AKR animals contained minimal amounts of glycoprotein in embryos, increased concentrations in the newborn, and high levels in the adult spleen; the major internal protein was present in low levels throughout embryogenesis and in the newborn and rose to high levels in the adult spleen. The results indicated that genetic differences in oncornavirus protein expression are evident during embryogenesis as well as in the adult animals and that this protein expression may be influenced by the state of differentiation of the cell. Our observations do not support the concept that oncornavirus gene expression plays an important role in mammalian embryogenesis.
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