Alternative splicing:

Several cases  are  now  known  where  variant  tissues  splice  the  main  RNA transcript of a single gene through instead pathways, where the exons which are lost and those which are retained in the last 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 certain  splice sites to direct the splicing  pathway  selected. These instead splicing pathways are very important still they allow cells to synthesize a range of functionally distinct proteins from the main transcript of a single gene.

Figure:  Alternative RNA splicing pathways. In the simple example shown, the transcript can be spliced by alternative pathways leading to two mRNAs with different coding capacities, i.e. exons 1, 2 and 3 or just exons 1 and 3. For genes containing many exons, a substantial number of alternative splice pathways may exist which are capable of generating many possible mRNAs from the single gene.