Tropomyosin 3 encoded by the TPM3 gene is a member of the acting binding tropomyosin family, a component of the sarcomeric thin filaments troponin tropomyosin complex that is essential in muscle contraction by regulating the calcium dependent binding of the myosin head to the actin filament in anticipation of the force generating power stroke. Mutations in TPM3 cause a clinical and histopathological heterogeneous group of neuromuscular disorders characterized by progressive weakness that includes CAP myopathy, congenital fiber type disproportion and nemaline myopathy. Recent studies of the deltaK7 mutation in TPM2 suggest that the pathogenic mechanism underlying tropomyosin related disease with progressive muscle contractures may be a gain of function change, leading to elevated Ca2+ sensitivity of force generation, while this has not yet been shown conclusively for TPM3. Here we report two unrelated patients with a de novo TPM3 single glutamic acid deletions resulting in a much more significant contractile phenotype with marked congenital muscle stiffness associated with ventilator failure in one case. We hypothesize that these single amino acid deletions result in increased Ca2+ sensitivity of the troponin tropomyosin complex and a consequently hypercontractile sarcomere. We thus expand the clinical, phenotypic and pathophysiological spectrum of TPM3 mutations, which now join recessive αB-crystallin mutations and specific ACTA1 and TPM2 mutations as a myogenic cause for neonatal muscle rigidity.