For safety, any resistance gene introduced into the germline should be regulated so that it is expressed only where it is needed, namely in the T helper cells and their relatives. It seems unlikely that the sorts of molecules used in this case would be harmful to other cells, but the first safety principle for germline genetic engineering should be to express a new or altered gene in the minimum range of cells needed. Genes are highly amenable to regulation. There are many different classes of regulatory elements. The best understood is the promoter, a short stretch of DNA at the beginning of a gene.
Promoters are attachment sites for special proteins that enable the gene to be expressed.
These proteins are called transcription factors because transcription is the first step in gene expression. Every promoter requires one or more particular transcription factor that specifically recognizes that promoter, attaches to it and initiates the transcription process. If a cell does not make a particular transcription factor, the genes that are dependent on it will not be expressed. Our genome codes for thousands of different transcription factors. Each cell type makes its own unique subset of them for the proteins it needs.