Provirus Integration at the 3 Region of N‐myc in Cell Lines Established from Thymic Lymphomas Spontaneously Formed in AKR Mice and a [(BALB/c × B6)F(1)AKR] Bone Marrow Chimera

Among 18 thymic leukemia cell lines which have been established from spontaneous thymic lym‐phomas in AKR mice as well as in bone marrow chimeras which were constructed by transplanting allogeneic bone marrow cells into irradiated AKR mice, three proviral integration sites were identified; near c‐myc, N‐myc and pim‐l loci. No integration site specific for chimeric leukemia cell lines was found. In three thymic leukemia cell lines which contained rearranged N‐myc, genes, insertions of long terminal repeats (LTRs) of murine leukemia viruses were detected at 18 or 20 bp downstream of the translational termination codon. These results demonstrate that the 3’region of the N‐myc gene is one of the integration targets for murine leukemia viruses in spontaneous thymic lymphomas. In these three cell lines, N‐myc mRNA was stably transcribed and transcription of c‐myc mRNA was down‐regulated. The integrated murine leukemia viruses in AKR thymic leukemia were most likely AKV, though the DNA sequence of the LTR inserted in the genome of a leukemic cell line from [(BALB/c × B(6))F(1)‐AKR], CAK20, was different from LTRs of murine leukemia viruses so far reported.