The rapid evolution of tools for genome editing has created a dizzying array of possibilities for novel therapeutic strategies, even though to date only a handful of clinical applications have been realised. Proof-of-concept demonstrations of targeted genome modification in vitro and in small animal models of inherited single gene disorders have to be translated into effective therapies. Interest has naturally gravitated towards opportunities for collection, ex vivo modification and return of blood, immune and stem cells. Initial applications designed to modify T cells to protect against HIV or to confer potent anti-leukaemic effects have reached clinical phase, and further applications to modify blood stem cells are close to being applied. There are generic considerations of safety, on- and off-target effects and possible genotoxicity as well as issues relating to more sophisticated systemic approaches where niche occupation and host immunity become relevant. Such issues will be likely addressed over time, with carefully designed clinical trials required to determine therapeutic risks and benefits.