HP and Parkinson disease: For example, it has been reported that plasma transferrin level correlates with the tremor-dominant phenotype of Parkinson’s disease (Si et al., 2018), as well as the abnormally low iron in PD patients was especially pronounced for subjects of haptoglobin Hp 2-1 phenotype, which also have higher risk of PD (Costa-Mallen et al., 2015).