GDNF and Parkinson disease: In the field of adult stem cells, valproic acid was shown to cooperate in DA differentiation of human spermatogonial stem cells when added to a conditioned medium from olfactory ectomesenchymal cells together with the small molecules SHH, RA, SB431542, FGF8, TGF-β3, and GDNF: the derived DA neurons cells were able to engraft and survive in a PD mouse model, partially rescuing the PD phenotype [34].