In its various embodiments, the invention provides, first, a composition comprising a vector for transfecting a cell. The vector comprises a first nucleic acid encoding an antisense agent having thereon an RNA interference target for a transcript of a gene endogenous to the cell. The vector further comprises a second nucleic acid that encodes a cell-killing agent. The second nucleic acid further comprises a sequence of nucleotides transcribable into a non-coding region of a transcript of the second nucleic acid, such that the non-coding region becomes an RNA interference target for the antisense agent. In the transfected cell, the vector operates to interfere with the expression of the cell-killing agent unless and until the vector senses certain endogenous gene signals, whereupon it releases the cell-killing agent. Second, the invention provides a method of treating a disease in a patient by killing cells responsible for the disease, the method comprising administering the vector to the patient until the disease, or a symptom thereof, is ameliorated.