Schematic representation of the luciferase-prothrombin fusion proteins. (A) Luciferase-prothrombin fusion protein translated from a nonspliced mRNA. Absence of splicing leads to the production of a luciferase protein with a tail derived from exon 13 and part of intron M of prothrombin. (B) Luciferase-prothrombin fusion protein translated from a spliced mRNA. In the event of correct splicing, a luciferase protein connected to a protrombin-derived tail is formed. The prothrombin tail is produced from the translation of exon 13 and 14. aa indicates amino acids.