Various forms of alternative splicing. The introns are marked in blue with the exons marked in red, purple, yellow and green as the sit in order in the gene. (A) Exon skipping occurs when an exon is excluded. (B) Mutually exclusive exons implies that one exon is included while another is excised (C and D). Alternative splice sites involve the 5’ and 3’ exons being shortened due to internal splice sites. (E) Intron retention occurs when an intron is retained and not excised. (F) Alternative polyadenylation sites can change the length of the 3’ untranslated region. (G) Alternate promoters can change transcription initiation sites.