Analysis of the translational efficiency of TPO P1 transcripts in vitro and in transfected COS cells. Annotation as in Fig4. (A through C) Full-length P1 transcript (P1-wt); (D through F) transcripts with exon 2 skipping (P1▵E2). (A and D) Exon composition of the TPO mRNA isoforms. Open rectangle, GUG repeat. (B and E) In vitro transcription translation analysis. (Top) ▵GUG, mRNA with deletion of the cryptic GUG initiation sites. (Bottom) uAUG 4 and GUG, proteins with extended amino terminus through in-frame initiation at the fourth uAUG or the GUG repeat, respectively. (C and F) TPO production and secretion by transfected COS cells.