OPRM1 and leukemia: In contrast, overexpression of this type of receptor has been observed in leukaemia cells (Table 4), and the use of opioid agonists has even been proposed for the treatment of different types of tumours, including those of leukaemia [135]; in this sense, methadone, a specific ligand of MOR, has been proposed for the treatment of cancer [135, 136] because it induces apoptosis and increases the sensitivity of leukaemia cells to the effect of doxorubicin in a mechanism that involves the reduction of cAMP, a promoter of cell proliferation [124].