OPRK1 and breast cancer: Genes coexpressed with DPP6 were involved in cell Kappa-type opioid receptor-related pathways and networks, such as “Muscle contraction_Role of kappa-type opioid receptor in heart”, “Development_Schema: FGF signaling in embryonic stem cell self-renewal and differentiation”, and “Neurophysiological process_Kappa-type opioid receptor signaling in the central nervous system” playing essential roles in breast cancer patients (Supplementary Figure S5, Table S3).