chain and then hn chain of the neurotoxin is amplified from the cdna by the polymerase chain reaction pcr using appropriate oligonucleotide primers whose sequences are designed based on the bont a neurotoxin cdna sequence of seq id no 9 these procedures are performed using the standard techniques of molecular biology as detailed in for example sambrook et al already incorporated by reference herein the primer defining the beginning of the coding region 5 u2032 side of the l chain fragment is given a stui site the pcr primer defining the 3 u2032 end of the hn encoding domain has the following features from 3 u2032 to 5 u2032 a 5 u2032 region sufficiently complementary to the 3 u2032 end of the hn encoding domain to anneal thereto under amplification conditions a nucleotide sequence encoding the human immunoglobulin hinge region u03b3 1 seq id no 11 a nucleotide sequence encoding the human cck 8 octapeptide seq id no 6 and a unique restriction endonuclease cleavage site the pcr product termed bont a l hn u03b3 cck is purified by agarose gel electrophoresis and cloned into a pbluescript ii sk vector the resulting plasmid is used to transform competent e coli cells and a preparation of the resulting plasmid is made the bont a l hn u03b3 cck fragment is excised from the pbluescript vector and cloned into a mammalian expression vector immediately downstream of a strong promoter the resulting vector is used to transfect a culture of the appropriate host cell which is then grown to confluence expression of the bont a l hn u03b3 cck polypeptide is induced and the cells are lysed the polypeptide is first purified by gel exclusion chromatography the fractions containing the recombinant therapeutic agent are pooled then the bont a l hn.