MPL transcript levels in platelets by semiquantitative RT-PCR.MPL mRNA was amplified from leukocyte-depleted platelets after reverse transcription with primers specific to the P isoform. Amplification of GAPDH was used as internal control. A ratio between them was calculated by densitometry after electrophoresis in 2% agarose gel and staining with ethidium bromide. (A) ▪ represents MPL/GAPDH ratio in healthy controls while ▴ indicates MPL/GAPDH ratio in patients. Results represent the average of 2 experiments. (B) Representative RT-PCR assay. MPL and GAPDH were amplified from sequential 1:5, 1:10, 1:20, and 1:40 cDNA dilutions from a healthy control and patient II-2, yielding 354-bp and 554-bp products, respectively. MPL transcripts are expressed at lower levels in the patient's sample. A 100-bp DNA ladder is shown on the left lane.