The decline in Poseida Therapeutics' stock price following the FDA's Orphan Drug Designation might seem counterintuitive, as regulatory milestones are generally seen as positive. However, market reactions can be influenced by various factors including investor expectations, the broader market environment, or the specifics of the designation itself. In this case, the immediate market reaction is negative, possibly due to concerns over the commercial potential of the drug or the costs and challenges ahead in further development stages.