MKI67 and neoplasm: In contrast, expression of the cell proliferation marker Ki67 in PINK1-/- tumors was reduced compared to WT, while expression of Ki67 in PINK1-/-plus huPINK1 tumors that over-expressed huPINK1 was increased, indicating reversible inhibition of tumor cell proliferation in vivo by ablation of PINK1 (Figure 3B).