**4.2 Insertion site of HTLV-1**

At present, in addition to the carcinogenesis by the proteins of these viruses themselves, attention has been focused on the activation of host genes at the insertion site. The integrated HTLV-1 provirus has the LTR [35] in both directions for the transcription. This sequence has many motifs that bind to transcription factors on the host and have strong promoter/enhancer activities. Therefore, there may be an increase in constitutive expression of the gene at the insertion site. Ogawa performed systematic genome analysis of ATL cells in 426 ATL patients and showed that genes interacting with Tax, T-cell receptor-NF-κB signal transduction, T-cell transport, and other T-cell-related pathways as well as genes related to immune surveillance are injured by the integration. Also, the expressions of VAV1, IRF4, and FYN related to lymphocyte maturation and signal transduction, chemokine receptors CCR4 and CCR7, and gene fusions (CTLA4-CD28 and ICOS-CD28) were enhanced in the infected cells [37]. Viral transcripts were mainly derived from the antisense strand. Also, the suppression of *Tax* expression and the constitutive expression of *HBZ* were observed in almost all patients. These insertion sites have not been known to be likely to occur in any particular sequence motif.

### **4.3 Clinical test for HTLV-1 detection**

To date, no chromosomal abnormality specific to HTLV-1 has been observed, and Southern blot or inverse PCR methods are used for diagnosis; however, in recent years, multiplex LAMP (RT-LAMP) using universal probe has been developed. The method detects both HIV and HTLV-1 RNA from the same sample [39]. HAS HTLV-1 analyzing system (HAS)-Flow method has also been proposed to evaluate the progression of ATL stage by applying flow cytometric analysis focusing on surface markers of infected immune cells [38].
