Differential promoter usage and alternative splicing of TPO pre-mRNA. (A) Ribonuclease protection analysis of human liver RNA. (Top) Lanes 1 and 2, undigested riboprobe and tRNA control. P1 transcripts (P1-wt) account for approximately 10% of TPO mRNA. Note that P2 transcripts (P2-wt) initiate at multiple start sites. In vitro transcribed sense mRNAs corresponding to different 5′-UTR were used as RNA size markers (lanes 4 through 6). Numbers at the right indicate length of the RNA size markers in nucleotides. (Bottom) Length and position of the riboprobe (thick line) and the protected fragments (arrows) with respect to the TPO mRNA 5′-end. (B) Assessment of exon 2 skipping (P1▵E2) by ribonuclease protection. Annotation as in (A).