The derivation of two mRNA from the same gene encoding two amylases is shown in Figure 29.10. It may be noted that exon L is actually derived from a part of the intron of the amylase gene tor salivary glands, (ii) During expression of immunoglobulin genes, substitution of one exon by another occurs at the 3' end of the transcript, resulting in proteins having different amino acid sequences at the C-terminal ends, (iii) During adenovirus late expression, the same transcript gives several mature mRNAs, each having the same leader sequence (not to be translated into protein product) derived from all the three exons (exons 1, 2 and 3) and a coding sequence derived from only one of the three available coding exons C₁ C₂ and C₃ (Fig. 29.11). (iv) In troponin T gene for rat muscle, at the 3' end there are five exons, of which only four each take part in forming mature mRNAs for two different forms of troponin T. The two forms differ, depending upon which of the two alternative exons (α or β) is used to construct mRNA, the other exons (W, X, Z) being common (Fig. 29.12).