Kleefstra syndrome (KLEFS1) is a rare genetic neurodevelopmental disorder affecting multiple body systems. It continues to be under-researched, and its prevalence remains unknown. This paper builds on the international KLEFS1 cohort of 172 individuals based on the caregiver-reported outcomes collected within the online data collection platform GenIDA and reports the occurrence, frequency and severity of symptoms in KLEFS1. The study clearly shows the importance of caregiver-reported outcomes collections in the rare disease domain. Moreover, the study emphasizes the need for more specific and enhanced data collection methods, suggesting recommendations to optimize caregiver-reported registries and foster an even more profound understanding of rare diseases.