The 3′flanking sequence of the human prothrombin gene determines low efficiency of 3′end formation. (A) Arrangement of the tandem constructs. (B) Northern blot of cytoplasmic RNA of cells transfected with constructs shown in Figure 5A. The numbers shown below the panel represent the ratio of mRNA processed at the 5′ site relative to that processed at the 3′ site. The signals were quantified by phosphoimaging.