A 25-year-old man presents 1 week after discovering that his left testicle is twice the normal size. Physical examination reveals a nontender, testicular mass that cannot be transilluminated. Serum levels of alpha-fetoprotein and human chorionic gonadotropin are normal. A hemiorchiectomy is performed, and histologic examination of the surgical specimen shows the cells grow inalveolar or tubular patterns, the neoplastic cells have an epithelial appearance,exhibit hyperchromatic nuclei with prominent nucleoli.. Compared to normal adult somatic cells, this germ cell neoplasm would most likely show high levels of expression of which of the following proteins?