A number of medical disorders are caused by either an insufficiency of a gene product or a defective gene product. Gene therapy can be used to provide a sufficient amount of a gene product when a disorder is caused by an insufficiency and can also be used to inactivate genes that produce defective gene products. Examples of disorders that can be treated by providing a sufficient amount of a gene product include lysosomal storage diseases, clotting disorders, diabetes, and alpha-1 antitrypsin deficiency. Systems and methods to produce B cells that express selected antibodies and gene products are described. The systems and methods can be used to provide prolonged and tunable expression of the gene products for the treatment of diseases such as lysosomal storage diseases, clotting disorders, diabetes, or other protein deficiencies.