The human T cell lymphotropic virus type 1 (HTLV-1) infects 5 to 10 million individuals and remains without specific treatment. This retrovirus genome is composed of the genes gag, pol, env, and a region known as pX. This region contains four open reading frames (ORFs) that encode specific proteins. The ORF-I produces the protein p12 and its cleavage product, p8. In this study, we analyzed the genetic diversity of 32 ORF-I sequences from patients with different clinical profiles. Seven amino acid changes with frequency over 5% were identified: G29S, P34L, L55F, F61L, S63P, F78L, and S91P. The identification of regions where the posttranslational sites were identified showed a high identity among the sequences and the amino acid changes exclusive of specific clinical profile were found in less than 5% of the samples. We compare the findings with 2.406 sequences available in GenBank. The low overall genetic diversity found suggested that this region could be used in the HTLV-1 vaccine development.