Enhancing the capacity of preclinical studies to predict efficacy in humans would be a welcome move and its usefulness would reach beyond stem cell-based trials. Such an adaptation, however, would be incomplete as regards the more specific field pf cellular transplantation studies. In addition to concerns regarding predictions of efficacy, preclinical benchmarks aimed at assessing safety also require adaptation –and rethinking- in order to successfully do their work in the area of stem cell-based interventions. For example, hESC-based intervention involve risks that are potentially more durable and cannot be easily tamed through point-in-time validation of preclinical benchmarks. A further example is the concept of purity, which is crucial in the assessment of safety in preclinical drug assessment but cannot simply be transposed to stem cell-based interventions. Rather than true preclinical criteria, such benchmarks should be viewed as proxies intended to assess different components of safety and efficacy in preparing for research with human subjects.