Spinal muscular atrophy is a neuromuscular disorder caused by a mutation in the survival of SMN1 gene. Diagnosis of the disease is based mainly on the presence of hypotonia and symmetrical [1]. A five-month-old male with SMA type 1 was admitted to the Children’s Neurology Clinic for gene therapy with onasemnogene abeparvovec. He was diagnosed with spinal muscular atrophy in newborn screening.Neurological examination of the patient revealed abolition of deep tendon reflexes. Administration of the medication proceeded without complications. Two weeks after gene therapy, abnormal test results were observed. The patient experienced several adverse effects of the therapy, which indicated thrombotic microangiopathy (TMA). Gene therapy with onasemnogene abeparvovec provides many hopes for patients with SMA. On the other hand, its safety remains uncertain and patients require comprehensive long-term monitoring for possible side-effects.