RET and Parkinson disease: In this review, we will: provide an overview of the bone morphogenetic proteins (BMPs) and their intracellular signalling effectors, the Smads; review the evidence that these morphogens play an endogenous role in mDA neurons; review the effects of BMPs on mDA neuron survival and axon growth; and highlight the potential of application of BMP ligands or molecular targeting of Ret-independent BMP–Smad signalling as a potential neuroprotective approach to attenuate or stop the degeneration of A9 mDA neurons in PD.