1.   This arrangement of exons presumably arises as a result of the positive pressure to select this particular splicing pathway in visceral smooth muscle cells.

2.   Furthermore the data suggests that an additional level of complexity characterises regulation of MHC expression in that a positive pressure is required for selection of the visceral splicing pathway.

3.   Scheme showing alternative splicing pathway of the visceral isoform.

4.   Dotted lines show the likely splicing pathways producing the vascular and visceral-specific isoforms.