Severe combined immunodeficiency due to adenosine Deaminase deficiency (ADA-SCID) is an ultra rare yet fatal disease. Without prompt treatment, children born with ADA-SCID die within the first year of life. The incidence is estimated fewer than 50 children per year in the US and EU combined (Verbsky, 2012; Vogel, 2014). Strimvelis® is a stem cell gene therapy created for an individual patient from their own cells intended to correct the root cause of the disease. In April 2016 GSK received a positive CHMP opinion recommending marketing authorization for Strimvelis® for ADA-SCID, which was only 11 months post submission. In this talk, experience from the submission and regulatory interactions will be shared from a statistical perspective with a focus on the collaboration and the scientific nature of the communications between the industry and the EMA agency. The talk will also cover the interactions with FDA so far where they have acknowledged challenges in ultra-rare diseases and are willing to take reasonable approaches for advanced therapies.