Discloses the use of a modified an HIV TAT OYI protein or variant (SEQ ID NO: 1 or 2) in the manufacture of a medicament for preventing or treating acquired immunodeficiency syndrome (AIDS). The SEQ ID NOs: 1 or 2 both contain a modification of serine to cysteine amino acid at position 22 in the sequence. Further discloses that SEQ ID NO:2 comprises N or C terminal truncation.