SMAD6 and Parkinson disease: In support of a similar function in humans, the expression of BMP2 showed the highest correlation with five mDA-specific marker genes in the human SNc and BMP2 transcript levels were downregulated in late-stage PD (Goulding et al., 2019), whereas the protein levels of the inhibitory SMAD6 appeared to be upregulated in the SNc of PD patients (Vitic et al., 2021).