EXHIBIT 99.A ------------ AGREEMENT TO FILE JOINTLY We, the undersigned, hereby express our agreement that the attached Schedule 13G and all subsequent amendments thereto executed by both of us are filed on behalf of each of us. HOECHST MARION...

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CUSIP No.  893735 10 0             13G

                              EXHIBIT 99.A
                              ------------


                      AGREEMENT TO FILE JOINTLY


     We, the undersigned, hereby express our agreement that the attached
Schedule 13G and all subsequent amendments thereto executed by both of us
are filed on behalf of each of us.


                                    HOECHST XXXXXX XXXXXXX, INC.

       March 17, 1998                /s/ Xxxxxxx X. Xxxxxx
---------------------------         -------------------------------------
          (Date)                       (Name and Title)
                                     Xxxxxxx X. Xxxxxx
                                     Vice President and Secretary
                                     
                                     HMR PHARMA, INC.

        March 17, 1998               /s/ Xxxxxxx X. Xxxxxx
---------------------------         -------------------------------------
          (Date)                        (Name and Title)                   
                                     Xxxxxxx X. Xxxxxx
                                     Vice President and Secretary