The cDNA encoding the equine copper/zinc superoxide dismutase (SOD1) was cloned from leukocyte total RNA from healthy horses and its nucleotide (nt) sequence was determined. We further sequenced the SOD1 gene from 16 horses diagnosed with equine motor neuron disease (EMND) and eight unrelated, clinically normal horses to determine if this disease, similar to amyotrophic lateral sclerosis (ALS) in humans, is linked to SOD1 mutations. The 465-bp SOD1 coding region in the horse encodes 153 amino acid (aa) residues. Equine SOD1 exhibited 81.8 and 79.9% sequence identity to the human homolog at the nt and aa levels, respectively, with only five distinct aa in the two loops that constitute the active site of the enzyme. None of the human SOD1 mutations found in the familial form of ALS were detected in SOD1 of the 16 affected horses. Although DNA sequence analysis identified three potential polymorphisms in equine SOD1, these were silent and were found in both normal and EMND-afflicted horses. At this time, there is no conclusive evidence for EMND linkage to SOD1 mutations.