The identification of human genes associated with bone fragility started around the 1990s through the study of monogenic syndromes with marked skeletal phenotypes such as osteogenesis imperfecta due to COL1A1 and COL1A2 defects (9) and osteopetrosis due to TCIRG1 defects (10). The gene discussed is TCIRG1; the disease is osteopetrosis.