Calpainopathy is the most prevalent limb girdle muscular dystrophy (previously LGMD2A) caused by mutations in the CAPN3 gene, the first LGMD gene to be identified. Until recently, LGMD2A was believed to be inherited in a purely autosomal recessive (AR) fashion. Now at least two autosomal dominant (AD) forms of calpainopathy have been described, including in-frame deletions and single base pair substitutions, suggesting both AR and AD inheritance. New nomenclature has been adopted to consider these modes of inheritance; LGMDR1 (AR) and LGMDD4 (AD). When compared to the typical course in LGMDR1, patients with LGMDD4 seem to have a later symptom onset and/or a milder phenotype. Here we describe a large family from US with a pathogenic mutation in a single allele of CAPN3 consistent with LGMDD4. Consistent with LGMDR1, the pattern of muscle loss includes marked atrophy of the posterior leg compartment. An international consortium of scientists identified several other LGMDD4 mutations with muscle biopsies revealing reduced active and inactive calpain-3 protein, with impaired activation or incomplete auto-proteolysis. Our projections of these collective LGMDD4 mutations onto a predicted crystal structure of calpain-3 reveals a hotspot of pathogenicity around the calmodulin-binding region at the central segment of the molecule. The significance of this region in causing autosomal dominant disease is not well understood, though prior studies showed calmodulin binding enhances calpain-3 activation. These autosomal dominant mutations appear to act in a dominant-negative fashion by loss of autoproteolytic function and likely destabilization of calpain-3 molecule. The natural history of calpainopathies has direct genotype-phenotype correlation, with two null mutations being more severe than two missense mutations, and one null/one missense shows a phenotype in between. Similarly, partial retention of the functional protein from the intact allele in LGMDD4 may be the reason for slower disease progression compared to LGMDR1 with nearly identical pathology.