Q. A 22-year-old man presents to the clinic complaining of persistent swelling on the back of his ankles. He says that these swellings have been increasing in size over the last six months. He also mentions a family history of premature heart disease. On physical examination, you notice bilateral, firm, painless nodules over his Achilles' tendons. Additionally, there's a noticeable grayish-white ring around the periphery of his corneas. What is the underlying molecular abnormality in this patient's condition?