Here we demonstrate that muSRK-015P (SRK-015P with a mouse IgG1 framework to reduce the potential for immunogenicity) improves muscle mass and function in two variants of the SMNΔ7 mouse model of SMA which have been pharmacologically adapted to rescue SMN deficiency either at day 1 or day 24, thus representing early or late therapeutic intervention. Here, SMN2 is linked to proximal spinal muscular atrophy.