Table 2 summarizes the results of studies on candidate genes for ET that had been previously related to PD. Although a number of studies showed weak associations between several variants in certain genes with the risk for ET, replication studies did not confirm these associations. Replication studies on the possible contribution of several allelic variants in the genes CYP2C19, CYP2C9/CYP2C8, RIT2, and IL1B, which have shown an association with the risk for ET in single studies, are lacking. Here, CYP2C9 is linked to essential thrombocythemia.