Objective: This study was undertaken to determine whether a low residual quantity of dystrophin protein is associated with delayed clinical milestones in patients with DMD mutations. Methods: We performed a retrospective multicentric cohort study by using molecular and clinical data from patients with DMD mutations registered in the Universal Mutation Database–DMD France database. Patients with intronic, splice site, or nonsense DMD mutations, with available muscle biopsy Western blot data, were included irrespective of whether they presented with severe Duchenne muscular dystrophy (DMD) or milder Becker muscular dystrophy (BMD). Patients were separated into 3 groups based on dystrophin protein levels. Clinical outcomes were ages at appearance of first symptoms; loss of ambulation; fall in vital capacity and left ventricular ejection fraction; interventions such as spinal fusion, tracheostomy, and noninvasive ventilation; and death. Results: Of 3,880 patients with DMD mutations, 90 with mutations of interest were included. Forty-two patients expressed no dystrophin (group A), and 31 of 42 (74%) developed DMD. Thirty-four patients had dystrophin quantities < 5% (group B), and 21 of 34 (61%) developed BMD. Fourteen patients had dystrophin quantities ≥ 5% (group C), and all but 4 who lost ambulation beyond 24 years of age were ambulant. Dystrophin quantities of <5%, as low as <0.5%, were associated with milder phenotype for most of the evaluated clinical outcomes, including age at loss of ambulation (p < 0.001). Interpretation: Very low residual dystrophin protein quantity can cause a shift in disease phenotype from DMD toward BMD. ANN NEUROL 2021;89:280–292.