Gómez-Duarte OG, Galen J, Chatfield SN, Rappuoli R, Eidels L, Levine MM. Expression of fragment C of tetanus toxin fused to a carboxyl-terminal fragment of diphtheria toxin in Salmonella typhi CVD 908 vaccine strain. Vaccine. 1995 Nov; 13(16):1596-602.