Those following Bluebird Bio’s rise to market glory will be familiar with the reality of gene therapy investing. In this brave new world clinical data are dribbled out patient by patient in the form of case reports, and company valuations fluctuate wildly based merely on the expression of a target enzyme. On top of this expectations are through the roof (Gene therapy deal-making shows no signs of stopping, September 24, 2018). The example of Avrobio shows the risk: this company’s market cap had exceeded $1.2bn without it ever revealing hard clinical data, and when the first 18-month case report with the Fabry disease gene therapy AVR-RD-01 was at last presented yesterday the stock crashed 52%. The problem is not that AVR-RD-01 did not work, but that the levels of AGA enzyme being produced by the patient fell markedly six months after infusion of the therapy. The patient was able to discontinue enzyme replacement, but Avrobio’s message is “one-time gene therapy”, so waning activity cuts little ice. Investors in Sangamo will now wait to see if this company’s Fabry disease gene therapy, ST-920, can do better.