Recessive mutations in ALS2 (juvenile amyotrophic lateral sclerosis) are causative for early-onset upper motor neuron diseases, including infantile ascending hereditary spastic paralysis (IAHSP). The goal of this study is to identify novel disease-causing ALS2 mutations. Mutations in ALS2 were screened by direct sequencing of complementary DNA obtained from patients' lymphoblasts. We report a novel ALS2 missense mutation in patients affected by IAHSP. This homozygous G669A mutation in exon 4 is predicted to result in a tyrosine substitution at cysteine 156 of the RCC1 (regulator of chromatin condensation)-like domain, encoding a putative guanine exchange factor for Ran guanosine triphosphatase, leading to a loss of ALS2 function due to instability of mutant protein. These results highlight the important role of the RCC1-like domain in ALS2 stability and function that is essential for upper motor neuron maintenance. Ann Neurol 2006;59:976-980.