HTLV-II tax Gene: DNA sequences that form the coding region for at least three proteins which regulate the expression of HUMAN T-LYMPHOTROPIC VIRUS 1 and HUMAN T-LYMPHOTROPIC VIRUS 2. The proteins are p21(x), p27(rex), and p40(tax). The tax (trans-activator x) and rex (regulator x) genes are part of pX but are in overlapping reading frames. X was the original designation for the sequences or region (at that time of unknown function) in the long open reading frame (lor) which is now called pX.