Immunohistochemistry showing cyclin A1 expression in tissue sections. (A) Bone marrow biopsy from a case of acute promyelocytic leukemia (AML M3). Promyelocytes show strong nuclear staining for cyclin A1. (B) Normal bone marrow biopsy with negative or focal weak staining of myeloid precursors. (C) Adenocarcinoma of the colon with malignant cells showing nuclear staining for cyclin A1 (arrowheads). Hematoxylin counterstain original magnification × 400.