Comparison of Mpl levels in MKs in the presence or absence of TPO. MKs obtained in the presence of TPO (3 days) were either maintained in TPO (right lane) or shifted to a medium without TPO and containing SCF, IL-6, and IL-3 (left lane). Immunoblot analysis of MK lysates using antibodies directed against Mpl, GpIIb, P-selectin, and actin, as described in Materials and Methods.