HTRA2 and essential thrombocythemia: HTRA2, which encodes a serine protease, plays a role in the pathogenesis of genetic type ET (119, 120), and its homozygous allele is involved in the pathogenesis of PD (121), providing genetic evidence of a link between the two disorders, although an Asian study did not find a role for this gene in ET-PD (122, 123).