1. A viral expression vector whose genome comprises: (a) the human INSM1 promoter; (b) a pair of tandem nicotinic acetylcholine receptor neuronal restrictive silencer elements downstream from said promoter; (c) an insulator element upstream from said promoter; and (d) one or more DNA sequences encoding a polypeptide toxin, or encoding a polypeptide reporter molecule, or encoding both a polypeptide toxin and a polypeptide reporter molecule; wherein: (i) said viral expression vector is competent to infect at least some mammalian cells; (ii) said insulator is adapted to block the influence of viral regulatory elements on the transcription within mammalian cells of said one or more DNA sequences, but not to affect the influence of said promoter or the influence of said neuronal restrictive silencer elements on the transcription of said one or more DNA sequences; (iii) said promoter is operatively linked to said one or more DNA sequences, and is adapted to cause the selective transcription of said one or more DNA sequences in tumor cells of neuroendocrine origin; (iv) said tandem neuronal restrictive silencer elements are operatively linked to said promoter, and are adapted to selectively repress the transcription of said one or more DNA sequences in non-neuronal cells; (v) the reporter molecule, if expressed, facilitates the identification of cells in which the reporter molecule is expressed; and the toxin, if expressed, is lethal or conditionally lethal to cells in which the toxin is expressed; whereby: said viral expression vector is adapted to be administered to a mammal in vivo, and to cause the selective in vivo expression of said one or more DNA sequences in tumor cells of neuroendocrine origin, without expression of said one or more DNA sequences in non-neuronal cells; whereby the reporter molecule, if expressed, facilitates the identification of tumor cells of neuroendocrine origin; and whereby the toxin, if expressed, is lethal or conditionally lethal to tumor cells of neuroendocrine origin.