The Federal Circuit explained as follows:
In re Baxter International highlights ex parte reexamination is not subject to a legal estoppel from the court decision such as applies to inter partes reexamination. Further, it reminds that patent challengers have a lower evidentiary standard in ex parte reexamination than in federal court. In ex parte reexamination, the challenger must only prove invalidity by a preponderance (i.e., it's more likely than not); in court, the challenger must prove invalidity by clear and convincing evidence. Accordingly, ex parte reexamination may be preferably over litigation if you seek to prove a US patent is invalid over written prior art.
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