p21 and Ki-67 expression in megakaryocytes. Serial sections of BM from an ET patient were immunostained for p21 (A) and Ki-67 (B). Positive cells showed red nuclear staining. Arrowheads indicate megakaryocytes: (▪) strongly positive; (▩) moderately positive; (▨) weakly positive; (□) negative (original magnification × 50). (C) Percentage of Ki-67 negative (−) and weakly (+−), moderately (+), and strongly (++) positive megakaryocytes subdivided according to p21 positivity ([−] none, [+−] weak, [+] moderate, [++] strong). Numbers of counted cells are shown in parentheses.