BUSINESS LAWexpansion mergers. The relevant market test is different than in monopolization cases; in a Section 7 action, relevance of market may be proved.In assessing horizontal mergers, the courts will look to the market shares of emerging companies, industry concentration ratios, and trends toward concentration in the industry.To prove a Section 7 case, the plaintiff must show that the merger forecloses competition “in a substantial share of” a substantial market. Conglomerate merger cases are harder to prove and require a showing of specific potential effects, such as raising barriers to entry into an industry and thus entrenching monopoly, or eliminating potential competition. Joint ventures may also be condemned by Section 7. The Hart-Scott-Rodino Antitrust Improvements Act of 1976 requires certain companies to get premerger notice to the Justice Department.8. CasesHorizontal Restraints of TradeNational Society of Professional Engineers v. United States435 U.S. 679 (1978)This is a civil antitrust case brought by the United States to nullify an association’s canon of ethics prohibiting competitive bidding by its members. The question is whether the canon may be justified under the Sherman Act, 15 U.S. c. § 1 et seq. (1976 ed.), because it was adopted by members of a learned profession for the purpose of minimizing the riskthat competition would produce inferior engineering work endangering the public safety. The District Court rejected this justification without making any findings on the likelihood that competition would produce the dire consequences foreseen by the association. The Court of Appeals affirmed. We granted certiorari to decide whether the District Court should have considered the factual basis for the proffered justification before rejecting it. Because we are satisfied that the asserted defense rests on a fundamental misunderstanding of the Rule of Reason frequently applied in antitrust litigation, we affirm.