Cumulatively, these data demonstrate that CELF1 up-regulation is not a unique diagnostic marker of DM1 muscle, and that this disease also induces an increase in HNRNPA1, and possibly other nuclear RNA binding proteins, that modulate the effects of MBNL sequestration and DM1-induced spliceopathy. Here, MBNL1 is linked to myotonic dystrophy type 1.