Walsh et al. assessed the levels of serotonin in the blood and of 5-HIAA in plasma and urine, as well as bone density and bone turnover markers, OST, PINP, and C-terminal cross-linked telopeptide of type I collagen alpha chain (CTX) in patients with carcinoid syndrome and in a group of healthy people [32]. The gene discussed is CYP27A1; the disease is carcinoid syndrome.