A 22-year-old woman is diagnosed with bipolar disorder. Over the next year, she develops neurologic manifestations that include resting and intention tremors, rigidity, chorea, dysphagia, and dysarthria. On physical examination, she has bilateral Babinski signs. There are ring-like deposits of green material involving the cornea bilaterally, but her vision is not decreased. One year later, she has an illness that lasts 3 weeks, with nausea, vomiting, and malaise and scleral icterus. Laboratory findings include serum AST, 100 U/L; ALT, 122 U/L; alkaline phosphatase, 105 U/L; total bilirubin, 4.5 mg/ dL; glucose, 77 mg/dL; and creatinine, 0.9 mg/dL. Serologic test results for hepatitis A, B, and C are negative. This episode subsides without treatment, but she eventually develops cirrhosis. A mutation in a gene encoding for what substance is most likely to be present in this woman?