The pX protein of HTLV-I is a transcriptional activator of its long terminal repeats.

Expression of the pX protein of human T-cell leukemia virus type I (HTLV-I) in animal cells demonstrates that this protein is a specific transcriptional activator of the long terminal repeats (LTR) of HTLV-I. Several other promoters are not affected by pX. No lymphocyte-specific factors are required for this activation. pX can be detected in the nucleus of transfected monkey kidney cells (line CV1) by indirect immunofluorescence. These results indicate that the pX protein is essential for the replication cycle of the virus and that it may be directly involved in the immortalization of human lymphocytes by HTLV-I.