The emergence of concomitant trials for multiple investigational drug products for the treatment 21 of rare diseases can pose significant challenges to effective drug development due to the limited 22 number of patients worldwide with any given rare condition. The purpose of this guidance is to 23 facilitate drug development in pediatric rare diseases. In particular, it discusses a new possible 24 approach to enhance the efficiency of drug development in pediatric rare diseases using Gaucher 25 disease as an example. This new approach consists of a controlled, multi-arm, multi-company 26 clinical trial, which aims to facilitate the development of multiple drug products in a time27 efficient manner while minimizing the number of patients necessary to be treated with placebo. 28 The general principles presented should be viewed as a proposal only, and the principles 29 underlying the proposal may be extended to other areas of drug development in rare diseases. Of 30 note, the specific recommendations regarding drug development for Gaucher Disease apply only 31 to systemic (i.e., non-neurological) manifestations of Gaucher disease in treatment-naïve patients 32 with Type I and Type III phenotypes, across all the pediatric ages (i.e., up to 18 years of age).