The objectives of the study are to provide long term expression of IDUA and improve the current clinical outcome of enzyme replacement therapy (ERT) or hematopoietic stem cell transplantation (HSCT) therapy in subjects with attenuated MPS I, a recessive lysosomal storage disorder that results from mutations in the gene encoding IDUA. SB-318 is a therapeutic for ZFN-mediated genome editing which will be delivered by adeno-associated virus (AAV)-derived vectors. SB-318 is intended to function by placement of the corrective copy of the IDUA transgene into the genome of the subject's own hepatocytes, under the control of the highly expressed endogenous albumin locus, and is expected to provide permanent, liver-specific expression of iduronidase for the lifetime of an MPS I patient.