Abstract : Background: Whereas mutations affecting the helical domain of type I procollagen classically cause Osteogenesis Imperfecta (OI), helical mutations near the amino (N)-proteinase cleavage site have been suggested to result in a mixed OI/Ehlers-Danlos syndrome (EDS)-phenotype. Methods. We performed biochemical and molecular analysis of type I (pro-) collagen in a cohort of seven patients referred with a clinical diagnosis of EDS and showing only subtle signs of OI. Transmission electron microscopy of the dermis was available for one patient. Results: All of these patients harboured a COL1A1 / COL1A2 mutation residing within the most N-terminal part of the type I collagen helix. These mutations affect the rate of type I collagen N-propeptide cleavage and disturb normal collagen fibrillogenesis. Importantly, patients with this type of mutation do not show a typical OI phenotype but mainly present as EDS patients displaying severe joint hyperlaxity, soft and hyperextensible skin, abnormal wound healing, easy bruising, and sometimes signs of arterial fragility. In addition, they show subtle signs of OI including blue sclerae, relatively short stature and osteopenia or fractures. Conclusion: Recognition of this distinct phenotype is important for accurate genetic counselling, clinical management and surveillance, particularly in relation to the potential risk for vascular rupture associated with these mutations. Because these patients present clinical overlap with other EDS subtypes, biochemical collagen analysis is necessary to establish the correct diagnosis. © 2013 Malfait et al.; licensee BioMed Central Ltd.