 The mutation is caused by a frameshift mutation caused by one of two deletions in the ALAS2 exon 11, either c. 1706-1709 delAGTG or c. 1699-1700 delAT.This alters the 19th and 20th residues of the C-terminal domain thereby altering the secondary structure of the enzyme.The delAT mutation only occurred in one family studied whereas the delAGTG mutation occurred in several genetically distinct families.