The human T-cell leukemia/lymphoma virus (HTLV) genome, in addition to the structural Gag and Env proteins and retroviral enzymes, carries a region at its 3 0 end originally designated pX. To date, we know that this region encodes two essential transcriptional and posttranscriptional positive regulators of viral expression, the Tax and Rex proteins, respectively (reviewed elsewhere in this issue). Here, we will review current knowledge of the functions of three additional proteins encoded in the pX region, p12 I , p13 II , and p30 II .