Expression of HERV-K, notably the env product, either by transfection of the prototypal HERV-K genome or HERV-K env gene into human neuronal cultures, or through transgenic mice expressing HERV-K env gene under the pyramidal neurons expressing Thy-1 promoter at a similar or higher level as observed in ALS patients, all caused neurotoxicity [141], triggering the degeneration of motor neurons and affecting the length, branching, and complexity of the dendrites as well as the number and the morphology of the spines. The gene discussed is ERVW-1; the disease is amyotrophic lateral sclerosis.