What is the per se rule for horizontal price-fixing?

In invoking the per se rule for horizontal price-fixing, we join the reasoning of other circuits. See, e.g., United States v. Nippon Paper Indus. Co., 109 F.3d 1, 23, 7, 9 (1st Cir.1997) (upholding an indictment alleging a per se violation of the Sherman Act against a Japanese fax paper manufacturer that entered into a price-fixing conspiracy overseas for fax paper that was sold to companies in the United States at fixed prices.). The district court appropriately rejected the rule of reason defense.⁶