p21^waf1/cip1 mRNA is induced at very early stage of megakaryocytic differentiation. (A) UT-7 and HL-60 cells were cultured with 10 ng/mL of TPA for 24 hours. RNA was isolated at the given time points and subjected to Northern blot analysis for p21^waf1/cip1 mRNA expression. EtBr-stained 28S and 18S rRNAs are shown as a loading control. (B) UT-7 cells were cultured in the absence (none) or presence of TPA, nocodazole, or a combination of both. p21 mRNA expression was examined after 24 hours of culture.