Significantly higher serum (Mateo et al., 2010) and salivary HO-1 levels (Song et al., 2018) have been reported in patients with PD than in normal controls, prompting the hypothesis that HO-1 levels in biological fluids represent a promising marker of PD and may be used to assess the extent of neurodegeneration. Here, HMOX1 is linked to Parkinson disease.