CXCR4 and neoplasm: The importance of BMs is also suggested by in vitro studies, which underline that NEN cells exhibit osteotropism [23,112] and that the mechanisms involved in the formation of BMs, including the EMT and the alteration of RANKL/OPG and CXCR4/CXCL12 pathway, are similar to those found in other tumor types [39,40,41].