In conclusion, we demonstrate that COL1A1 and COL1A2 single exon skips and missense mutations, resulting in glycine substitutions, which are located in the type I collagen helical domain near the N-propeptide cleavage site, affect proper N-propeptide processing and result in a mixed OI/EDS phenotype. Here, COL1A1 is linked to osteogenesis imperfecta.