37 CFR 1.414: The United States Patent and Trademark Office as a Designated Office or Elected Office.

(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) National stage processing for international applications entering the national stage under 35 U.S.C. 371.