During enzyme replacement therapy with PEG-ADA, ADA activity ranged from undetectable to 14 555 nmol x [s.sup.-1] x [(L plasma).sup.-1].
Except in those few cases where bone marrow from an HLA/MLR-compatible sibling donor has been available, therapy for ADA deficiency has been based on enzyme replacement with PEG-modified ADA and, since 1990, on gene therapy transfer, the latter requiring concomitant PEG-ADA therapy until recently (24).