These include: 1) the demonstration that elevated luminal concentrations of substance P (secreted from mucosal sources) are only measured in 12% of patients [45]; 2) fasting circulating substance P concentrations are elevated in <20% of carcinoids [46]; and 3) at least two distinct serotonin producing NET lesions have been identified – serotonin producing NETs in the pancreas are TAC1/substance P negative [47]. The gene discussed is TAC1; the disease is carcinoid tumor.