alternative mRNA splicing, via spliceosome GO:0000380 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. 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. alternative nuclear mRNA splicing, via spliceosome biological_process splice site selection