The report dates back to 2010, when a general practitioner in the Ain department notified the Rhône-Alpes Regional Health Agency (ARS) of a suspected spatio-temporal cluster of transverse agenesis of the upper limbs involving three cases among his patients, all born in 2010 in three neighboring municipalities within the Ain department. The InVS Unit in the Rhône-Alpes Region (Cire RA) was contacted and called upon the Rhône-Alpes Birth Defects Registry (REMERA) to conduct a joint investigation. This initial investigation did not allow for a conclusion regarding the existence of a cluster because, on the one hand, only 2 out of 3 cases were located and interviewed, and on the other hand, the calculation of excess cases could not be performed due to the lack of data on the number of live births in the Ain department. On October 3, 2014, the president of REMERA reported to the Rhône-Alpes Regional Health Agency (ARS) new cases of transverse agenesis of the upper limbs geographically close to the first cases reported in 2010, in the department of Ain. The ARS and the Cire organized a meeting with REMERA on October 17, 2014, to share the details of the report and define the actions to be taken as part of a joint investigation. During a debriefing organized by Santé publique France in September 2016 regarding three investigations into suspected clusters of limb agenesis occurring in three different regions, REMERA reported another case involving a child born in late 2014 in the Ain department.