A Clinical Study to Enable Process Validation of Commercial Grade OTL-101

Inclusion Criteria: * Provision of written informed consent by the subject or parent(s)/legal guardian(s), prior to any study related procedures taking place. Where consent is provided by the parent(s)/legal guardian(s), assent by the subject should also be sought, if appropriate * Age ≥30 days and \<18 years * Diagnosis of ADA-SCID based on either: * 1\) Evidence of ADA deficiency, defined as Decreased ADA enzymatic activity in erythrocytes, leukocytes, skin fibroblasts, or in cultured fetal cells to levels consistent with ADA-SCID as determined by the reference laboratory OR Identified mutations in ADA alleles consistent with a severe reduction in ADA activity * 2\) Evidence of ADA-SCID, defined as Family history of a first order relative with ADA deficiency and clinical and laboratory evidence of severe immunologic deficiency OR Evidence of severe immunologic deficiency in subjects prior to the institution of immune restorative therapy, based on at least one of the following: * Lymphopenia (absolute lymphocyte count \<400 cells/μL) OR absence or low number of T cells (absolute CD3+ count \< 300 cells/μL) * Severely decreased T lymphocyte blastogenic responses to phytohemagglutinin (either \<10% of lower limit of normal controls for the diagnostic laboratory, or \<10% of the response of the normal control of the day, or stimulation index \<10) * Identification of SCID by neonatal screening revealing low T cell receptor excision circles (TREC) levels * Ine…