There's a number of ways that you can control the expression of an introduced gene via chemical induction. That is, you can put a gene into cells that has an expression promoter that can turn on when you add a specific compound. I haven't done it myself, but it's fairly common. There are even strains of mice that have genes that can be turned on and off chemically. One strategy would be to turn on a gene that induced apoptosis in this manner. Thus, the cells self-destruct when exposed to a certain compound. Introducing a gene into cultured cells is very easy, and there are a number of ways to do it. Basically, you force the cells to take up a ring of DNA that has the gene you want to introduce a small number of these cells will incorporate the DNA into their genome, or you construct a virus that introduces the gene. If you want to keep only those cells expressing your gene, you introduce the gene with another one that confers resistance to a compound that kills the cells. You add the compound, and the only cells that survive are those that incorporated your DNA.