Structure of Tax1, Tax2B and their mutant proteins. The amino acid sequence of PBM and its mutants are indicated. The tax1, tax2B genes and their mutant genes inserted in the pHβ Pr-1-neo expression plasmid have been described previously . To convert the expression plasmid from pHβPr-1-neo to pβA-IRESpuro plasmid (pβAIP), an EcoRI-BamHI fragment containing the β-actin promoter of pHβPr-1-neo was inserted into the NruI-BamHI site of pIRESpuro3 (BD Biosciences) by a blunt-end ligation. Then, wild type tax or tax mutant cDNAs were inserted into the BamHI site of pβAIP.