Experimental litters subsequently consist of ∼50% control littermates (Smn+/−; SMN2tg/0, total one allele mouse Smn and two copies of SMN2) and ∼50% severe SMA-like littermates (Smn−/−; SMN2tg/0, no mouse Smn and two copies of SMN2). Here, SMN1 is linked to proximal spinal muscular atrophy.