Spinocerebellar ataxia-21 (SCA21) is an autosomal dominant neurologic disorder characterized by onset in the first decades of life of slowly progressive cerebellar ataxia, which is associated with cognitive impairment in most patients (summary by Delplanque et al., 2014). For a general discussion of autosomal dominant spinocerebellar ataxia, see SCA1 (164400).

Tolchin-Le Caignec syndrome (TOLCAS) is a developmental disorder characterized by mildly to moderately impaired intellectual development and behavioral problems, such as autism, ADHD, labile mood, and aggressive episodes. Many patients have bony abnormalities, including osteochondroma, craniosynostosis, dysmorphic facies, arachnodactyly, and large head circumference. Rarely, additional congenital anomalies may also be observed. These additional features and the bony defects are highly variable (summary by Tolchin et al., 2020).