37 C.F.R. § 1.414 The United States Patent and Trademark Office as a Designated Office or Elected Office.
Title 37 - Patents, Trademarks, and Copyrights
(a) The United States Patent and Trademark Office will act as a Designated Office or Elected Office for international applications in which the United States of America has been designated or elected as a State in which patent protection is desired. (b) The United States Patent and Trademark Office, when acting as a Designated Office or Elected Office during international processing will be identified by the full title “United States Designated Office” or by the abbreviation “DO/US” or by the full title “United States Elected Office” or by the abbreviation “EO/US”. (c) The major functions of the United States Designated Office or Elected Office in respect to international applications in which the United States of America has been designated or elected, include: (1) Receiving various notifications throughout the international stage and (2) Accepting for national stage examination international applications which satisfy the requirements of 35 U.S.C. 371. [52 FR 20047, May 28, 1987]
Title 37: Patents, Trademarks, and Copyrights
PART 1—RULES OF PRACTICE IN PATENT CASES
Subpart C—International Processing Provisions
General Information
§ 1.414 The United States Patent and Trademark Office as a Designated Office or Elected Office.

