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SUMMARYRetrovirus infection of cultured murine teratocarcinoma cells depends upon the state of differentiation. We have used two cell lines derived from a teratocarcinoma of mouse, strain 129. One, an undifferentiated pluripotential cell line (PCC4), is restrictive to viral infection, while the other, a differentiated myoblast-derived cell line (PCD1), is fully permissive to virus replication. We have shown that no virus RNA expression can be found in PCC4 cells 48 h post-infection and that no nucleic acid sequences can be found in an integrated form in PCC4 cells. However, the kinetics of formation of free proviral intermediates show that the three forms (I, II and III) of free virus DNA are synthesized in both PCC4 and PCD1. Free proviral DNA disappears gradually after 24 h in PCC4 cells while all forms increase in PCD1. These results suggest that the viral multiplication restriction occurs somewhere between the proviral DNA synthesis and integration of DNA in the cellular genome.

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