On day six of treatment, genetic panel testing using the Oncomine Dx Target Test, next-generation sequencing-based companion diagnostic approved by Japan’s Pharmaceuticals and Medical Devices Agency (PMDA) for detecting RET gene alterations in thyroid cancer, identified a RET M918T mutation, prompting a switch to selpercatinib at 320 mg daily, which led to a rapid and marked reduction in cervical lymphadenopathy and serum calcitonin within approximately one week, demonstrating the potent therapeutic efficacy of RET-targeted therapy. The gene discussed is RET; the disease is thyroid gland carcinoma.