Engineering the bone marrow of an adult AIDS patient to produce new T cells is an extraordinarily ambitious goal and might not be possible at all in view of the complex way that mature T cells are formed. It would be far more feasible to introduce the resistance gene into the germline. This would prevent AIDS rather than treat it. 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.