High Aurora-B/AIM-1 protein level is only detected in transgenic, small, CD41⁺ megakaryocytes. Immunohistochemical staining of bone marrow cells isolated from wild-type or transgenic mice. Aurora-B/AIM-1 was stained with rhodamine-conjugated antibody (red). The megakaryocyte surface marker CD41 was stained with FITC (green). Cell nuclei were stained with Hoechst (blue). Large and small CD41⁺ cells (megakaryocytes) are indicated with arrows. Small CD41⁺ cells have diameters in the range observed for all other bone marrow cells (noted by phase-contrast microscopy). Small megakaryocytes in transgenic mice showed strong, diffuse Aurora-B/AIM-1 staining. Original magnification, × 200.