definiens: shall mean a legal opinion that satisfies each of the following conditions : ( a ) is addressed to CUBIST for CUBIST ’s benefit, ( b ) provi des that CUBIST ’s exercise of its rights and licens es under, and its activities under and pursuant to, th is Agreement, to Obtain Regulatory Approval, make, have made, use, promote, market, sell, have sold, offer to sell, import, export, and Commercialize HepeX - B, would not infringe or misappropriate the intellectual pro perty rights addressed in that opinion ( including b y literal infringement and infringement under the doctrine of equivalents and/or other applicable legal standard s ), or that such intellectual property rights are invalid or un enforceable, ( c ) is provided by a nationally recogn ized United States law firm reasonably acceptable to both Parti es, and ( d ) was obtained in connection with a joint defense agreement pursuant to which such nationally recogni zed law firm enters into an attorney - client relatio nship with both XTL and CUBIST for the purpose of providing su ch legal opinion