TPO-signaling proteins in CB-MKs and PB-MKs. Total levels of proteins related to TPO signaling were measured by Western blot in unstimulated mature MKs derived from CB and PB progenitor cells (after 14 days of culture with TPO only). C-MPL, S6K, and p-4E-BP-1 were present at significantly higher levels in CB-MKs compared with PB-MKs. There were no significant differences in JAK2, LNK, and ERK1/2 levels. The bars represent the means ± SEM of protein levels, expressed as a ratio to β-actin, in 4 independent experiments. *P < .05.