TI - Results Functional Domains of HTLV-1 Rex . AB - Mutational analyses permitted the assignment of functional properties to distinct domains of the Rex-1 protein (Fig 1A) . In addition to the characterized nuclear localization signal/RNA binding domain , central core activation domain , two multimerization domains , and the newly identified C-terminal stability domain , three PHOSphorylation sites have been identified at Ser-70 , Ser-177 and Thr-174 by the use of reverse-phase HPLC and sequential Edman degradation [39] . However , this approach only provided limited mapping coverage of Rex-1 and the functional relevance of the identified sites were not addressed . To date , no further studies have examined the possibility of other PHOSphorylation events or the effect of these post-translational modifications on Rex-1 function .