A follow-up plan for the index patient and her family members carrying RET D898Y should consist of yearly checkups, including assaying for parathyroid hormone, carcinoembryonic antigen, calcitonin, 24-hour urine catecholamine, and VMA levels, as well as screening using thyroid ultrasound to detect PCC, MTC, or hyperparathyroidism. The gene discussed is RET; the disease is hyperparathyroidism.