CELF1 and myotonic dystrophy type 1: The pathological CTG expansion in DMPK produces damage or dysfunction in various DM1 tissues because of the nuclear accumulation of cytosine–uracil–guanine (CUG) expanded pre-mRNAs transcribed from the mutant DMPK allele, eventually affecting expression and/or activity of specific RNA-binding proteins, named muscleblind-like splicing regulator (MBNL) and CUGBP Elav-like family member 1 (CELF1), that act as developmental regulators of alternative splicing of many different genes in several tissues.