A number of mouse models for spinal muscular atrophy (SMA) have been genetically engineered to recapitulate the severity of human SMA by using a targeted null mutation at the mouse Smn1 locus coupled with the transgenic addition of varying copy numbers of human SMN2 genes. Although this approach has been useful in modeling severe SMA and very mild SMA, a mouse model of the intermediate form of the disease would provide an additional research tool amenable for drug discovery. In addition, many of the previously engineered SMA strains are multi-allelic by design, containing a combination of transgenes and targeted mutations in the homozygous state, making further genetic manipulation difficult. A new genetic engineering approach was developed whereby variable numbers of SMN2 sequences were incorporated directly into the murine Smn1 locus. Using combinations of these alleles, we generated an allelic series of SMA mouse strains harboring no, one, two, three, four, five, six or eight copies of SMN2. We report here the characterization of SMA mutants in this series that displayed a range in disease severity from embryonic lethal to viable with mild neuromuscular deficits.