Abstract: The present invention is directed to a method for identifying a gene that regulates clostridial neurotoxin activity, the method comprising: a. providing a sample of human neuronal cells expressing a polypeptide that comprises a C-terminal detectable label, wherein the polypeptide is cleavable by a clostridial neurotoxin; b. altering expression of a target gene of the cells; c. contacting the cells with the clostridial neurotoxin; d. measuring an amount of C-terminal detectable label, thereby quantifying clostridial neurotoxin activity; and e. identifying the target gene as a regulator of clostridial neurotoxin activity when the quantified clostridial neurotoxin activity is different to the quantified clostridial neurotoxin activity when expression of the target gene is unaltered; or f.