Different modes of RNA splicing. a) Constitutive splicing is most common, where, as part of the normal processing of transcription, the spliceosome removes intronic (non-coding) portions of pre-mRNA. b) In diseased or abnormal cells, other pathways, such as aberrant splicing machinery could, lead to mutually exclusive splicing. c) Exon skipping or d) intron retention can also occur as part of normal splicing or in malignant cells treated with SPLMs.