We report the clinical and molecular characterization of 28 subjects with TCF20 de novo or inherited pathogenic single nucleotide variants/indels (SNV/indels) and 4 subjects with interstitial deletions involving TCF20. These subjects present with a core phenotype of DD/ID, dysmorphic facial features, congenital hypotonia, and variable neurological disturbances including ataxia, seizures, and movement disorders; some patients presented features including sleep issues resembling those observed in SMS. The gene discussed is TCF20; the disease is cerebellar ataxia.