A 41-year-old woman has had diminished hearing for the past 4 months. On physical examination, she has decreased hearing on the left. Sound lateralizes to the right ear on the Weber tuning fork test. A head MRI shows a sharply circumscribed, 4-cm mass adjacent to the left pons that extends toward the left inferior cerebellar hemisphere. A smaller 1-cm lesion is in a similar location on the right. Family screening reveals a similarly affected 38-year-old sibling. An inherited mutation involving which of the following genes is most likely to be present in this patient?