definiens: shall mean the compound known as L - FMAU, with the chemical name, 2 ' -fluoro-5 - methyl - B - L - arabinofuranosyluracil, including any salts and esters thereof . BUKW ANG acknowledges that viral resistance may limit the clinical utility and commercial potential of the Compounds . In the conduct of the BUKW ANG Development Program, BUKW ANG shall use its best efforts to conduct, or shall cause its sublicensee to use its best efforts to conduct, preclinical and clinical studies of the Compounds in combination with other anti - HBV drugs or drug candidates in order to detennine whether in vitro, synergy, pharmacologic compatibility, differing resistance patterns, chemical structures, mechanisms of action or other factors of the combination treatment regimens may limit or reduce CONFIDENTIAL TREATMENT REQUESTED 4 lO.2 - 4A4 BY PHARMASSET, INC . the development of resistance to the Compounds . BUKW ANG shall conduct, or shall cause its sublicensee to conduct, preclinical combination studies with at least two other drugs or drug candidates and shall use, or shall cause its sublicensee to use, its best efforts to conduct clinical combination studies with as many other drugs or drug candidates as it deems appropriate to determine the optimal combinations or sequence of combinations necessary to provide maximum efficacy against HBV and enhance the clinical utility and commercial potential of the Compounds . Such combination studies may include two or more drugs or drug candidates and various sequences of treatment regimens and, because the outcome of such combination studies can not be predicted with certainty, these studies may include other drugs or drug candidates being developed by BUKWANG, its sublicensee or third parties which because of various factors are ultimately determined not to be additive to, or synergistic with, the Compounds