cells ref name hillisetal k post transcriptional control mrna involve alternative splicing primary mrna transcripts resulting single gene giving rise different mature mrnas encode family different proteins ref name hillisetal k ref name freeman ref well studied example ''sxl'' gene ''drosophila'' determines sex animals gene contains four exons alternative splicing pre mrna transcript generate two active forms sxl protein female flies one inactive form protein males ref name hillisetal k another example human immunodeficiency virus hiv
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