Ann: MSB Licensee Files For Skin Disease Marketing Approval, page-6

Apparently "over a 16-year period (1986 through 2002), the prevalence of EB was estimated to be approximately 11 per million and the incidence approximately 20 per million live births" (https://www.ncbi.nlm.nih.gov/pubmed/27463098). This was in the US. The primary risk factor is family history of EB, but based on my (limited) research on BMJ, Dermnet, and Uptodate, there is no comment about a racial preponderance. Birth rates in Japan are just under 1 million (https://www.japantimes.co.jp/news/2018/12/21/national/number-babies-born-japan-2018-lowest-since-records-began-population-decline-highest/#.XJgx6ygzaUk). Based on this a very rough guide is that I wouldn't bank on any more than 20 new cases per year in Japan. I guess the question then becomes how many rounds of treatment, the frequency of the treatment, and the price of the treatment per patient.