alternative mRNA splicing, via spliceosome The process of generating multiple mRNA molecules from a given set of exons by differential use of exons from the primary transcript(s) to form multiple mature mRNAs that vary in their exon composition. biological_process alternative nuclear mRNA splicing, via spliceosome splice site selection Note that this process most commonly occurs in cis, selecting or skipping exons from the same primary transcript, but it has also been observed to occur in trans at low frequency, at least in some mammals. GO:0000380 mRNA splicing, via spliceosome