In contrast, BMI-1 overexpression triggers cell proliferation by repressing ink4a-ARF expression.124 BMI-1 is involved in gastric, pancreatic, breast and ovarian cancer among others.125–129 MEL-18 (PCGF2), a BMI-1 homolog, has a tumor suppressing activity.130–132 BMI-1 and MEL-18 expression levels are inversely correlated in various cancers.133,134 BMI-1 expression depends on its counterpart MEL-18 (Fig. 4a). The gene discussed is PCGF2; the disease is cancer.