Initially, toxic effects of the mutant 3′-UTR of DMPK, containing expanded CUG repeats, were investigated in several directions, including (a) examination of the intracellular properties of the mutant DMPK mRNA within DM1 cells; (b) identification of RNA-binding proteins, interacting with CUG repeats and (c) examination of the role of the mutant 3′-UTR of the DMPK mRNA in normal myogenesis in muscle cells and in mouse models. The gene discussed is DMPK; the disease is myotonic dystrophy type 1.