By Dr. Rao Vepachedu
35 USC section 121 provides a safe harbor from double patenting rejections against a patentee on claims that were a result of an earlier restriction requirement because it would be inequitable to reject divided claims on the ground of double patenting when the division was required on the theory that the original application contained claims for more than one invention.
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.
Section 121 is on its face inapplicable where a restriction requirement did not result in losing in the prior interferences.
 Pfizer, Inc. v. Teva Pharm. USA, Inc., 518 F.3d 1353, 1361 (Fed. Cir. 2008)
 35 USC § 121
 Biogen MA, Inc. v. Japanese Foundation for Cancer Research (Fed. Cir. 2015); available at: http://www.cafc.uscourts.gov/images/stories/opinions-orders/14-1525.Opinion.5-5-2015.1.PDF