Gene therapy may provide a long-term approach to the treatment of mucopolysaccharidoses. As a first step toward the development of an effective gene therapy for mucopolysaccharidosis type IVA (Morquio syndrome), a recombinant retroviral vector, LGSN, derived from the LXSN vector, containing a full-length human wild-type N-acetylgalactosamine-6-sulfate sulfatase (GALNS) cDNA, was produced. Severe Morquio and normal donor fibroblasts were transduced by LGSN. GALNS activity in both Morquio and normal transduced cells was several fold higher than normal values. To measure the variability of GALNS expression among different transduced cells, we transduced normal and Morquio lymphoblastoid B cells and PBLs, human keratinocytes, murine myoblasts C2C12, and rabbit synoviocytes HIG-82 with LGSN. In all cases, an increase of GALNS activity after transduction was measured. In Morquio cells co-cultivated with enzyme-deficient transduced cells, we demonstrated enzyme uptake and persistence of GALNS activity above normal levels for up to 6 days. The uptake was mannose-6-phosphate dependent. Furthermore, we achieved clear evidence that LGSN transduction of Morquio fibroblasts led to correction of the metabolic defect. These results provide the first evidence that GALNS may be delivered either locally or systematically by various cells in an ex vivo gene therapy of MPS IVA.