Reverse gene conversion can integrate SMN2 sequences into the SMN1 locus, resulting in an increased number of SMN1 copies; however, this can result in an increased risk of delayed-onset motor neuron disease [e.g., amyotrophic lateral sclerosis (ALS)], suggesting that excessive SMN expression can be harmful to motor neurons (10). The gene discussed is SMN2; the disease is amyotrophic lateral sclerosis.