Synthetic biology circuits to reverse senescence safely are being constructed. Cell senescence of progenitor and stem cells eliminates normal function. Senescence can be reversed by reducing expression of genes necessary for senescence maintenance. However, only senescence due to disorganization-based/aging-related causes, such as epigenetic drift or deregulated cell signalling should be reversed. Cells that are senescent due to mutations that activate proto-oncogenes or excessive double-stranded DNA breaks should remain in the senescent state or even be marked for destruction.