A very rare motor neuron disease with characteristics of progressive upper motor neuron dysfunction leading to loss of the ability to walk with wheelchair dependence and subsequently, loss of motor speech production. Affected patients are usually normal at birth and have normal early development. During the second year of life, they lose the ability to walk (some patients never walk due to early severe spasticity) and then develop slowly progressive upper motor neuron disorders including pseudobulbar palsy and spastic quadriplegia. Other signs include clumsiness, muscle weakness and balance difficulties. Mutations in the ALS2 gene (2q33-q35) encoding alsin, a protein that is abundant in motor neurons, and less commonly mutations in the ERLIN2 gene (8p11.2) have been reported. Inherited in an autosomal recessive manner.