Exon 7 was replaced with one in which a C to T transition, mimicking a mutation found in proximal spinal muscular atrophy (SMA) patients, results in exclusion of exon 7 from the premRNA. An FRT flanked neo cassette inserted upstream of exon 7 and was removed by flp mediated recombination. Reduced protein expression was confirmed by western blot analysis on kidney, liver, brain, spinal cord, and quadricep extracts.