This letter, published in the Journal of Neurology, discusses a case of myelitis as a side effect of tofersen therapy in a patient with SOD1-associated ALS. Tofersen is an antisense oligonucleotide that has been approved for the treatment of SOD1-associated ALS. The patient developed myelitis during therapy and experienced symptoms such as generalized myalgias and leg weakness. Treatment with prednisolone resulted in regression of symptoms, although leg weakness persisted. The case highlights the rare occurrence of myelitis as a side effect of tofersen therapy and suggests the need for closer observation of patients and attention to transient weakness after injections. The letter also mentions the potential for immunological or inflammatory responses and complications with the use of antisense oligonucleotides in neurological disorders. [Extracted from the article]