ACTA1 and Myotonia: Notably, a transgenic mouse model carrying the human skeletal actin gene (ACTA1) with long CTG repeats (HSALR mouse) is commonly used for preclinical studies because it expresses high levels of ACTA1 transcripts with 250 CUG repeats in the 3′ UTR, causing the sequestration of MBNL in myocytes as well as severe myotonia and progressive myopathy [15].