TH and Parkinson disease: Conversely, differentiation with factors such as the phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA) alone or in combination with RA have been shown to increase dopaminergic markers such as tyrosine hydroxylase (TH) and dopamine transporter (DAT) therefore increasing sensitivity to 1-methyl-4-phenylpyridinium (MPP+) commonly used to induce Parkinsonism symptoms [12–14][13].