One of the biggest challenges facing gene therapy is the development of vectors that direct the activity of therapeutic genes specifically to the sites of disease. To achieve this goal, the restriction of transgene transcription via synthetic promoters that are endowed with multiple specificities represents a particularly promising strategy. Towards this end, we have developed a generally applicable strategy (DCTF system) where a synthetic promoter is driven by an artificial heterodimeric transcription factor whose DNA-binding and transactivating subunits are expressed from two promoters with different selectivity. A crucial determinant of the DCTF system is the heterodimerization interface that should provide for a high affinity interaction without interference by endogenous proteins. Here, we describe such a dimerization system based on engineered Fos and Jun leucine zippers. We show the usefulness of this system for the combination of cell type-specific and cell cycle-regulated transcription and demonstrate its functionality in an in vivo setting.