In order to address this knowledge gap of HRU in rare diseases, we explored the relationship between HRU and diagnosis for individuals with 2 rare, pediatric-onset conditions: Kleefstra syndrome (KS; OMIM 610253) and SLC6A1 epileptic encephalopathy (SLC6A1; OMIM 616421). The gene discussed is SLC6A1; the disease is Kleefstra syndrome.