Abstract: The present disclosure provides sensitive and robust assays for determining the potency of payloads encoded by recombinant viral vectors. Particularly, the present disclosure provides assays to determine the potency of SMN polypeptide as expressed by recombinant viral vectors used for the treatment of spinal muscular atrophy. The present description encompasses, inter alia, methods for determining potency (e.g., biological activity), e.g., relative potency, of a recombinant viral vector.