Various cases  are  now  known  where  variant  tissues  splice  the  main  RNA transcript of a single gene by instead pathways, where the exons which are lost and those which are retained in the previous mRNA depend upon the pathway chosen that will shown in the figure. Presumably  some  tissues  hold  regulatory  proteins  which promote  or suppress  the use of particular  splice sites to direct the splicing  pathway  selected. These instead splicing pathways are very significant still they allow cells to synthesize a range of functionally distinct proteins from the major transcript of a single gene.