Scheme of the dual-reporter vector for assaying splicing efficiency. Vectors contained prothrombin intron M with the polymorphic 19911A>G site and adjacent exon sequence fused in-frame to β-galactosidase (β-gal) and luciferase reporter genes. In the case of inefficient splicing, a natural in-frame stop codon (X) is present within the intron. The ratio of luciferase activity to β-gal activity reflects the ratio of expression from spliced mRNA to the total expression of spliced and unspliced RNA. The parental plasmid is pTN23 (see “Materials and methods”).²⁶