This is an international, multi-center, randomized, prospective, double-blind, placebo-controlled, Phase 3 study, designed to assess the effect of EryDex (dexamethasone sodium phosphate [DSP] in autologous erythrocytes), administered by intravenous (IV) infusion once every 28 days, on neurological symptoms of patients with Ataxia Telangectasia (A-T).

The EryDex System (EDS) is a combination product that is used to load dexamethasone sodium phosphate (DSP) into autologous erythrocytes (EryDex) which is infused into the patient.

In the placebo arm, the subjects will receive autologous erythrocytes prepared with the EDS process using a placebo solution.

Upon completion of all screening assessments for eligibility, subjects meeting all selection criteria at baseline will be randomized in a 1:1 fashion to EryDex or placebo. Approximately 86 subjects 6- to 9-years-old, approximately 43 per group, will be randomized. Approximately 20 subjects 10 years of age and above, 10 per treatment group, may also be enrolled.