Japan’s regulatory authorities approved Guardant Health’s Guardant360 CDx as a companion diagnostic to detect ESR1 mutations and guide eligibility for Eli Lilly’s Inluriyo in breast cancer patients. The decision authorizes a plasma‑based CDx to identify genomic alterations for treatment selection. The approval expands regional access to liquid‑biopsy companion diagnostics and underscores the regulatory momentum for circulating tumor DNA tests as tools for therapy selection. Guardant’s clearance may accelerate adoption of plasma testing in clinical workflows and clinical trial enrollment strategies in Japan. Payers and oncology centers will evaluate implementation logistics, reimbursement and tissue‑vs‑plasma testing algorithms when adopting the CDx in routine practice.