Pharmaceutical firms enjoy market exclusivity for new drugs from concurrent patent protection and exclusivity of the clinical trials data submitted for market approval. Patent invalidation during drug development renders data exclusivity the sole source of protection and shifts the period of market exclusivity. In instrumental variables regressions, we quantify the effect of a one-year reduction in expected market exclusivity on the likelihood of drug commercialization. The effect is largely driven by patent invalidations early in the drug development process and by the responses of large originators. We provide estimates of the responsiveness of R&D investments to market exclusivity expectations.