In contrast to the apparently homogeneous viral population observed after tax sequencing, a significant degree of variability was seen with env. Indeed, there was an important sequence heterogeneity within each isolate, indicative of a quasispecies nature of HTLV-1/STLV-1 infections, as revealed by intrahost variations in the Env RBD (Figure 1). Here, ERVW-1 is linked to infection.