PDYN and cancer: Accordingly, this review aims to update the findings regarding the involvement of opioid peptides (enkephalins, endorphins, and dynorphins) in cancer development (e.g., mitogenesis, metastasis, angiogenesis); to suggest anticancer therapeutic strategies targeting the opioid peptidergic system (e.g., using opioid-receptor antagonists) and to highlight the main research lines to be developed in the future focused on the involvement of enkephalins, endorphins, and dynorphins in tumor development and progression.