DDC and carcinoid syndrome: However, whereas the “classical” carcinoid syndrome associated with midgut and hindgut carcinoids is mediated by 5-hydroxytryptamine (serotonin), the syndrome associated with ECL-cell tumors is “atypical” and is associated with 5-hydroxytryptophan, because of deficiency of the enzyme dopa-decarboxylase responsible for conversion of 5-hydroxytryptophan to 5-hydroxytryptamine.