Drug development for rare diseases is challenging because very few patients suffer from such diseases, and thus it is difficult to obtain relevant data. An international survey done with the EU Clinical Trials Register (EUCTR) and the Japanese Registry Network (JPRN) provided information on rare diseases and medicines. A total of 1,535 rare diseases and 1,539 drugs were studied in the total of 28,526 clinical trials examined. After analysis, it is clear that most diseases were only investigated in a limited number of studies; 70% of the diseases were studied in less than 10 studies, and 28% were tested in only one. It has also been established that most studies are about rare diseases associated with cancer. More research results can be found here.