Subclasses of opioids have been shown to have varying effects on cancer cells; specifically morphine has been shown to influence the proliferation and survival of cancer cells via direct effects on tumour cell DNA cleavage, Akt, PlK, MAPK, Src, GRB2-associated binding protein 1 (Gab-1) and STAT3 signalling pathways (70–72). Here, PLK1 is linked to cancer.