Organization of the HTLV-1 genome and expanded Rex coding region. The complete proviral genome is shown schematically. LTRs are depicted with their U3, R, and U5 regions. The location of the gag, pro, pol, env, tax, and rex genes and their corresponding reading frames are indicated along with ORF I and ORF II. Numbers below the genome denote kilobases. The genome containing the 2 rex coding exons has been expanded, and the location of Rex based on the nucleotide sequence of the HTLV-1 proviral clone Ach is presented. The nucleotide sequence around the Rex start site (ATG) for wtHTLV-1 and mutant HTLVRex^- proviral clone are indicated. Vertical arrows denote the location of Rex protein start sites (ATG) and the SphI diagnostic restriction enzyme site (CGATGC). Asterisk denotes stop codon (TAG) in HTLVRex^-. Location and orientation of oligonucleotide primers used in PCR and RT-PCR are indicated by horizontal arrows. Numbers below the Rex coding region denote amino acid number.