Murine leukemia viruses containing an approximately 30 S RNA subunit of unknown biological activity, in addition to the 38 S subunit of the viral genome.

The 50-70 S RNA complex of a clone of Kirsten murine leukemia virus (Ki-MLV), propagated in NIH 3T3 cells, as well as that of a Moloney (Mo)-MLV, propagated in a mouse thymus-bone marrow cell line, contained an approximately 30 S RNA of unknown biological activity in addition to 38 S genomic RNA. The approximately 30 S RNA shares with 38 S virion RNA the ability to form 50-70 S RNA complexes and to be packaged by virus particles. The morphology of the cell lines propagating these viruses was indistinguishable from that of uninfected controls.