Table 1.

Summary of expanded microsatellite disorders described in this review.

Disease	Repeat Sequence	Repeat Length	RNA structure	Location of Repeat	Primary Toxicity (RNA vs Protein)
Myotonic Dystrophy Type 1 (DM1)	CTG	Unaffected: 5–38 Affected: 50–5000	Single or multi-hairpin with U/U loops (Mooers et al., 2005; Kumar et al., 2011; Cruchten, Wieringa, & Wansink, 2019)	3’ UTR of DMPK gene (Brook et al.,1992)	RNA
Myotonic Dystrophy Type 2 (DM2)	CCTG	Unaffected: ~26 Affected: 75–11,000	Hairpin with CU/UC loops (Dere et al., 2004; Childs-Disney et al., 2013)	Intron 1 of CNBP gene (Liquori et al., 2001)	RNA
Fragile X-Associated Tremor/Ataxia Syndrome (FXTAS)	CGG	Unaffected: 6–54 Affected: 55–200	Hairpins with potential G-quadruplex (Handa et al., 2003; Blice-Baum & Mihailescu, 2014)	5’ UTR of FMR1 gene (Kremer et al., 1991)	RNA
Amyotrophic Lateral Sclerosis (ALS)	GGGGCC	Unaffected: ≤11 Affected: >30 and up to thousands	Hairpins and stable G-quadruplexes (Su et al., 2014; Balendra & Isaacs, 2018)	Intron 1 of C9orf72 gene (DeJesus-Hernandez et al., 2011)	Toxic protein production, RNA toxicity, and protein loss of function