Finally, candidate gene studies have not identified an association with ET risk for genes previously related with other degenerative diseases such as PD, idiopathic torsion dystonia, hereditary ataxias, or others, except for the findings on several variants of CYP2C19 [137], CYP2C9/CYP2C8 [145], RIT2 [158], and IL1B [147], and the increased risk for carriers of the ALAD rs1800435 variant in interaction with serum lead levels [178] or with a variant in the HMOX2 gene [179]. The gene discussed is HMOX2; the disease is Rare hereditary ataxia.