The Real Estate Settlement Procedures Act (RESPA), 12 U.S.C. § 2607(a), permitted title-services companies to pay certain referral fees within the real-estate-services industry only if three safe-harbor conditions were satisfied. In a policy statement, the Department of Housing and Urban Development (HUD) (plaintiff) added a fourth safe-harbor condition. Subsequently, real-estate agency Welles-Bowen Realty, Inc. (Welles-Bowen) (defendant) referred Erick Carter and other customers (customers) (plaintiffs) to an affiliated title-services company. The customers filed suit against Welles-Bowen in federal district court, arguing that Welles-Bowen had failed to satisfy the fourth safe-harbor condition. The district court ruled for Welles-Bowen. The customers appealed. HUD intervened, arguing that the policy statement was due Chevron deference as HUD’s reasonable interpretation of RESPA.