Splicing is dependent on the presence of the 7-methylguanosine cap.  A defect in splicing can occur as a result of mutation(s) in the guanylyltransferase, which can inhibit enzyme activity, preventing the formation of the cap. However the severity of the effect is dependent on the guanylyltransferase mutation.  Furthermore, the guanylyltransferase relieves transcriptional repression mediated by NELF.  NELF together with DSIF prevents transcription elongation.   Thus, mutations in the enzyme can affect transcription elongation.