Cells and cell lines are disclosed that are able to produce therapeutic proteins, antibodies, vectors, and viral vectors such as lentiviral vectors and adeno-associated viral (AAV) vectors. The cells and/or cell lines can have mutations or deletions in either one or both of the endogenous dihydrofolate reductase (DHFR-/-) or glutamine synthetase (GS-/-) genes such that DHFR and/or GS expression or function is substantially reduced or eliminated.