The initial event is thought to be integration near a cellular gene - generally myc. Viruses of this type are replication competent and the animals are chronically viraemic. So although many cells are infected, only one progresses to a tumour. Myc RNA is elevated 50x compared to normal cells. This suggests that the elevated myc transcription is due to the activity of the 3' LTR promoter elements.
Simplest interpretation is therefore that there is abnormal activation of the c-myc gene.
This is an example of promoter insertion. I and II.HTLV-1 is associated with specific lymphoid malignancies and is endemic in Japan, the Caribbean and Africa. The virus was isolated from an adult T cell malignancy. The malignancy is monoclonal. Unscreened blood transfusions Sharing of blood contaminated needles by intravenous drug users.