Ultragenyx Pharmaceutical and partner Mereo BioPharma announced the categorical failure of their late-stage candidate for a rare bone disease in a pivotal trial. The companies disclosed the negative readout Monday, prompting a sharp market reaction with Mereo shares plunging at market open. The failure eliminates a late-stage program and forces portfolio reassessment for both developers. The announcement provides immediate operational consequences: program termination, potential workforce and budgeting impacts, and reconsideration of regulatory and commercial plans tied to the asset. Ultragenyx and Mereo are the principal actors; the companies have not published detailed data in the release, leaving questions about safety, endpoints and subgroup signals unanswered. Investors and competing rare-disease developers will watch for follow-up data and management guidance on next steps.