Thus Yadav and colleagues(1) extended their findings in rodents to humans with LRP5 loss-of-function mutations, which cause the osteoporosis pseudoglioma (OPPG) syndrome characterized by severely reduced bone mass, and showed that serum serotonin levels were elevated at approximately 250 ng/mL in the sera of the three OPPG patients analyzed as compared with control subjects, who had a mean serum serotonin level of approximately 50 ng/mL. The gene discussed is LRP5; the disease is osteoporosis-pseudoglioma syndrome.