p21, p27, and Ki-67 staining of BM. Normal BM (A, B, and C) and BM from an ET patient (D, E, and F) was immunostained using anti-p21 antibody, 6B6 (A and D); anti-p27 antibody, G173-524 (B and E); and anti–Ki-67 antibody, MIB1 (C and F). Immunostaining was performed using the APAAP method, as described in Materials and Methods. Positive cells showed red nuclear staining. Although weak cytoplasmic staining in some megakaryocytes was visible when using the MoAb G173-524, it was not reproducible with another antibody F-8 (original magnification × 50).