Drug development is a costly and time-consuming process associated with a significant attrition rate in the clinic. Safety is an important factor in this, and is a particular issue for cell and gene therapies. A panel moderated by Dr John Maher of King’s College London and Leucid Bio, and featuring Dr Alastair King, Dr Francisca Neethling, and Dr Andrea Bisso, discuss this highly topical barrier to the further clinical and commercial success of the field.