The extra-skeletal manifestations of bone cell-related forms are still incompletely defined; with monoallelic WNT1 mutations patients have changes in spinal cartilaginous structures (63) and mild abnormalities in bone marrow hematopoiesis and reticulin formation (119), while in biallelic mutations the phenotype is more severe and OI-like but no bone marrow defects have been reported (55). The gene discussed is WNT1; the disease is osteogenesis imperfecta.