CCND2 and colorectal neuroendocrine tumor: We investigated differential gene expression signatures of metastatic vs non-metastatic colorectal NETs focusing on cell cycle regulatory gene set, and delineated six candidate genes for metastasis including ATM, CCND2, RBL2, CDKN3, CCNB1, and GTSE1. Among them, we have focused on ATM since ATM has been known as a tumor suppressor gene which encodes a principal DNA damage-signaling protein.