Transcripts from a single gene can undergo different splicing events. When mRNA is initially transcribed (known as pre-mRNA), it retains introns (thick black line), large segments of noncoding mRNA which separate exons, the coding regions. Through a process known as splicing, the introns are then removed and exons are ligated together to produce mature mRNA. Splicing also has the ability to remove exons or even retain introns resulting in the formation of different mature mRNA transcripts for the same gene (referred to as alternative splicing). Different mature mRNA transcripts encode for different proteins which may have alternative functions.