Long terminal repeat of Friend-MCF virus contains the sequence responsible for erythroid leukemia.

Friend-MCF virus induces erythroid leukemia when injected into newborn NFS mice whereas Moloney virus induces T-cell lymphoma. To identify the portion of Friend-MCF virus responsible for erythroid leukemia induction four in vitro recombinant viruses were constructed in which env regions or U3 regions of LTR were reciprocally exchanged between Friend-MCF and Moloney viruses. A FrMCF-Mol (LTR) virus whose genome was derived primarily from Friend-MCF virus together with 621 nucleotides of Moloney virus at its 3' end including the U3 region of LTR was a thymic lymphoma-inducing virus. A Mol-FrMCF (LTR) virus with the genome derived primarily from Moloney virus but 596 nucleotides of Friend-MCF virus information at the same region as FrMCF-Mol (LTR) was an erythroid leukemia-inducing virus. A Mol-FrMCF (env) virus whose genome was derived primarily from Moloney virus but which had 2.3 kbp of Friend MCF at the 3' end of the pol gene including most of the env gene with all of gp70 and the N terminal of p15E was a lymphoid leukemia-inducing mink cell focus-inducing virus. FrMCF-Mol (env) virus whose genome was derived primarily from Friend-MCF virus but had 2.7 kbp of Moloney virus at the same region as Mol-FrMCF (env) virus was an erythroid leukemia-inducing ecotropic virus. The Mol-FrMCF (LTR) and Mol-FrMCF (env) viruses induced mixed leukemia of erythroid and lymphoid cells in some mice.