MLK3 phosphorylates Pin1 and Ppia and induce PPIase activities. In breast cancer, MLK3 phosphorylates Pin1 and increases PPIase activity, and promotes Cdc2 mediated cancer cell proliferation. In T cells, MLK3 phosphorylates Ppia and increases PPIase activity and attenuates T cell effector function. Where, Cdc2, cell division cycle 2; MLK3, mixed lineage kinase 3; Pin1, peptidylprolyl cis/trans isomerase, NIMA-Interacting 1; Ppia, peptidylprolyl isomerase A.