Regarding motor neuron diseases and polyneuropathies (Fig. 2), HSP genes have been found mutated in patients with (i) peripheral nerve affections such as Charcot–Marie–Tooth (CMT) neuropathies, (ii) first and secondary motor neuron degeneration, such as ALS, and (iii) lower motor neuron disorders such as spinal muscular atrophy (SMA). The gene discussed is HSP90B2P; the disease is proximal spinal muscular atrophy.