The objective of this study is to determine the safety and efficacy of once weekly dosing of idursulfase-beta 0.5mg/kg administered by intravenous(IV) infusion for Hunter syndrome patients < 6 years old.

Inclusion Criteria:

1. The patient has a diagnosis of Hunter syndrome based upon biochemical criteria:

   • as measured in plasma, leukocytes, or fibroblasts,

     • a deficiency in iduronate-2-sulfatase (I2S) enzyme activity of ≤ 10 % of the lower limit of the normal range

   • That corresponds to one or more of the following:

     • a normal enzyme activity level of one other sulfatase
     • Confirmed as MPS2 by genetic test results
     • shows clinical symptoms/ visible signs of MPS2

2. < 6 years old and male

3. Patients who are able to comply with the study requirements

4. The patient's parent(s), or patient's legal guardian must have given voluntary written consent to participate in the study

Exclusion Criteria:

1. The patient has had a tracheostomy
2. The patient has known severe hypersensitivity or shock to any of the components of idursulfase
3. The patient has received treatment with another investigational therapy within 30 days prior to enrollment
4. History of a stem cell transplant
5. The patient has known severe hypersensitivity or shock to any of the components of test drug(excipient etc)