In the past, several plasma, serum and urine markers have been evaluated as predictors of tumor progression in NEN, such as chromogranin A (CgA) [10, 11], serotonin [12], 5-hydroxyindoleacetic acid (5-HIAA) [13], neurokinin A [14], neuropeptide K [15], neuron-specific enolase [16], or E-cadherin [17]. The gene discussed is TAC1; the disease is neoplasm.