The open reading frame I (ORF I)/ORF II part of the human T-cell leukemia virus type I X region is dispensable for p40(tax), p27(rex), or envelope expression

The X region of the human T-cell leukemia virus type I contains the second coding exon of the tax and rex regulatory proteins (open reading frame IV [ORF IV] and ORF III, respectively), as well as coding regions for more recently described proteins, p30(II) (or the tof protein) and p13(II) in ORF II and the putative rof protein and p12(I) in ORF I. Deletions and transcomplementation experiments showed that expression of the envelope, as well as that of the tax and rex proteins, was independent of the proteins encoded in the ORF I/ORF II region. Furthermore, p30(II) and p12(I) proteins could not replace the rex protein in a rex-dependent envelope or Gag protein expression system.