美國專利法第121條（分割案）

§121. Divisional applications

If two or more independent and distinct inventions are claimed in one application, the Director may require the application to be restricted to one of the inventions. If the other invention is made the subject of a divisional application which complies with the requirements of section 120 of this title [35 USC 120] it shall be entitled to the benefit of the filing date of the original application. A patent issuing on an application with respect to which a requirement for restriction under this section has been made, or on an application filed as a result of such a requirement, shall not be used as a reference either in the Patent and Trademark Office or in the courts against a divisional application or against the original application or any patent issued on either of them, if the divisional application is filed before the issuance of the patent on the other application. If a divisional application is directed solely to subject matter described and claimed in the original application as filed, the Director may dispense with signing and execution by the inventor. The validity of a patent shall not be questioned for failure of the Director to require the application to be restricted to one invention.

美國專利法第121條規範了分割申請案
因為在一個專利申請案中只能保護一個發明，若同時載有兩個或以上獨立或可區隔的發明，則會被要求分割處理，比如發出Restriction/Elections，要求申請人限制（restriction）保護範圍於其中之一發明。
而其他發明則可以分割案申請案處理，並應符合美國專利法第120條中「先前申請案與後續案」的規定，包括說明書揭露應符合112規定、若先前申請案未被專利、拋棄或是程序終結，則有前案申請日的好處等。
經過分割案處理的申請案並不能互相引用為核駁引證案。
另外，經要求分割時，不能質疑審查委員可能錯誤判斷限制（restriction）的要求。

Sec. 120. - Benefit of earlier filing date in the United States
An application for patent for an invention disclosed in the manner provided by the first paragraph of section 112 of this title in an application previously filed in the United States, or as provided by section 363 of this title, which is filed by an inventor or inventors named in the previously filed application shall have the same effect, as to such invention, as though filed on the date of the prior application, if filed before the patenting or abandonment of or termination of proceedings on the first application or on an application similarly entitled to the benefit of the filing date of the first application and if it contains or is amended to contain a specific reference to the earlier filed application. No application shall be entitled to the benefit of an earlier filed application under this section unless an amendment containing the specific reference to the earlier filed application is submitted at such time during the pendency of the application as required by the Director. The Director may consider the failure to submit such an amendment within that time period as a waiver of any benefit under this section. The Director may establish procedures, including the payment of a surcharge, to accept an unintentionally delayed submission of an amendment under this section

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