We postulate that ‘modifier genes’ are responsible for the variable disease severity in Osteogenesis Imperfecta (OI), by altering the manifestations of the major mutated gene. We aim to identify the modifier genes contributing to the intra-familial phenotypic variability in OI, using zebrafish models and human induced pluripotent stem cells. These ‘modifiers’ represent promising targets for intervening in disease initiation/progression.