To diagnose a NET of the prostate and distinguish it from a PCa with carcinoid-like features, the following criteria should be met: (1) not directly associated with a concurrent acinar adenocarcinoma of the prostate, (2) immunohistochemically positive for NE markers and negative for prostate-specific antigen (PSA), and (3) originating from the prostatic parenchyma. The gene discussed is KLK3; the disease is prostatitis.