Why do children cured by enzyme-replacement therapy for adenosine deaminase dediciency need periodic tratment ?

Explanation

Children cured by enzyme-replacement therapy for adenosine deaininase (ADA) deficiency need periodic treatment because it is not a completely curative method. In this Method, a functional ADA CDNA introduced into the patient's lymphocytes using retroviral vector. Since the lymphocytes have a definite life cycle, there is a n eed for periodic transfusion of the genetically engineered lymphocytes (having cDNA) into the patient.