Abstract Background The identification of a heterozygous exonic GGC repeat expansion in ZFHX3 underlying spinocerebellar ataxia type 4 (SCA4) has solved a 25‐year diagnostic conundrum. We used adaptive long‐read sequencing to decipher the pathogenic expansion in the index Utah family and an unrelated family from Iowa of Swedish ancestry. Contemporaneous to our discovery, other groups identified the same repeat expansion in affected individuals from Utah, Sweden, and Germany, highlighting the current pivotal time for detection of novel repeat expansion disorders. Methods Given that the pathogenic repeat expansion is rare on a population level, we proposed a common ancestor across all families. Here, we employed targeted long‐read sequencing through adaptive sampling, enriching for the chr16q22 region of interest. Results Using phased sequencing results from individuals from Utah, Iowa, and Southern Sweden, we confirmed a common ~2000‐year‐old ancestral haplotype harbouring the repeat expansion. Conclusion This study provides further insight into the genetic architecture of SCA4. © 2024 The Author(s). Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

The ZFHX3 GGC Repeat Expansion Underlying Spinocerebellar Ataxia Type 4 has a Common Ancestral Founder