CELF1 and myotonic dystrophy type 1: To further investigate the effects of HNRNPA1 on DM1 splicing targets, the splicing activity of HNRNPA1 and CELF1 proteins was also examined using mouse primary myoblasts transduced with lentiviruses overexpressing HNRNPA1 or CELF1 either prior to (day 0, D0) or after induction of myogenic differentiation into myotubes for days 3 to 7 (D3, D5, D7), D3-7.