Sisters aged 11 and 12 years at the time of examination with a single ATM gene pathogenic variant [c.1564_1565delGA (p.Glu522Ilefs43)] showed progressive ataxia, myoclonus, bulbar conjunctiva and skin telangiectasia, immunodeficiency on chronic intravenous immunoglobulins and recurrent skin infections. The gene discussed is ATM; the disease is immune system disorder.