Organization of the HTLV-4 genome (a) and schematic representation of the PCR-based genome walking strategy (b). (a) shown are non-coding long terminal repeats (LTR), coding regions for all major proteins (gag, group specific antigen; pro, protease; pol, polymerase; env, envelope; rex, regulator of expression; tax, transactivator), HTLV basic leucine zipper (HBZ), and 3' genomic open reading frames (ORF) of unknown function. Putative splice donor (sd) and splice acceptor (sa) sites are indicated. (b) Small proviral sequences (purple bars) were first amplified from each major gene region and the long terminal repeat using generic primers as described in methods. The complete proviral sequence was then obtained by using PCR primers located within each major gene region by genome walking as indicated with arrows and orange bars.