Rab27b is absent in p45 NF-E2^-/- MKs. (A) Immunoblot of wild-type and p45 NF-E2^-/- MKs using antibodies specific for the 6 Rab proteins known to be expressed in blood platelets or GAPDH and rabbit antimouse platelet serum (RAMPS) as loading controls. (B) Semiquantitative RT-PCR analysis shows Rab27b mRNA levels to be significantly reduced in the absence of NF-E2 function. The cDNA was synthesized from wild-type and p45 NF-E2^-/- MKs isolated over an albumin density gradient and amplified by PCR in the presence of α-[32P]dCTP radiotracer using primers specific for each Rab gene or for hypoxanthine phosphoribosyl transferase (HPRT). The number of PCR cycles was varied as indicated, and products were visualized by autoradiography. Results of both RT-PCR and immunoblot analysis are representative of 3 independent experiments.