Splicing of pre-mRNA
	introns are rare in yeast, but almost all mammalian genes are interrupted. a typical human gene has 7-8 exons over 15-20 kb; each exon is 100-200 bp long splicing takes place after transcription, capping, polyadenylation but before transport the spliceosome is another ribonucleoprotein the splice donor/acceptor sites are short; an additional branch site adds positioning information