What is Regulated splicing?

Alternative splicing produces different messengers from one gene by exon skipping, the choice between mutually exclusive exons, the retention of an intron, or the use of alternative 5$'$ or 3$'$ splice sites. It is regulated by RNA-binding proteins that bind sequence elements in the exons and introns near a splice site: SR proteins bound to exonic enhancers recruit the spliceosome and promote inclusion of the exon, hnRNP proteins bound to silencers block it, and the outcome in a given cell is set by the relative amounts of the two kinds. More than $90\,\%$ of human multi-exon genes are alternatively spliced; the Dscam gene of Drosophila, with four blocks of mutually exclusive exons (12, 48, 33 and 2 alternatives), can produce $38\,016$ distinct proteins, each neuron expressing a different set that lets its branches recognise and avoid one another.