While Env is detectable during early immortalization in vitro, viral sense transcription is silenced in vitro and in vivo through mechanisms such as 5′ LTR hypermethylation or deletion [136,137,138,139]; therefore, the contributions of Env to T-cell immortalization may occur during the early stages following initial infection, as it is likely remains inactive during HTLV-1 latency. Here, ERVW-1 is linked to infection.