GATA-1 levels in CB-MKs and PB-MKs. (A) The expression levels of multiple transcription factors related to MK development were analyzed by quantitative real-time PCR. Among the transcription factors studied, only GATA-1 was up-regulated in CB-MKs compared with PB-MKs. CB values were expressed as ratios to PB values. Bars represent the means ± SEM of 4 independent experiments. **P < .01. (B) Up-regulation of the GATA-1 protein was confirmed by Western blot analysis using an antibody targeting the N-terminal motif (no. 3535; Cell Signaling Technology), which only recognizes full-length GATA-1. *P < .05 (C) An antibody recognizing the C-terminal motif (SC-1233; Santa Cruz Biotechnology) was then used to differentiate between the full-length GATA-1 and GATA-1s. Only the levels of full-length GATA-1 were significantly higher in CB-MKs compared with PB-MKs. *P < .05. (D) The protein levels of cyclin D1, Cdk4, and p21 were also measured by Western blot analysis in human CB- and PB-derived mature MKs. Cyclin D1 protein levels were 2.6-fold higher in CB-MKs compared with PB-MKs, whereas Cdk4 and p21 protein levels were approximately 50%. In all cases, proteins were quantified by densitometric analysis of protein blots using the ImageJ program, and were expressed as a ratio to β-actin as an internal control. The bars represent the means ± SEM of 4 independent experiments, and the Western blots show representative results of each set of experiments. *P < .05.