Since five out of seven PD genes (α-SYNUCLEIN, PARKIN, UCHL1, PINK1 and LRRK2) plus the two latter genes (NURR1 and TAU) are expressed in midbrain dopaminergic neurons, possibly linking the abnormality in their expression or structure to selective degeneration of SNpc neurons, expression of genes within this neuronal population seems to be a suitable criterion for narrowing down the number of genes to be further analyzed for identification of PD genes to be associated with orphan PD loci. This evidence concerns the gene MAPT and Parkinson disease.