PPA1 and neoplasm: As described above, TGF‐β‐stimulated TβR‐I–TβR‐II‐mediated canonical signaling is responsible for potentiating TGF‐β growth inhibition mediated by TβR‐V by transcriptional activation of CDK inhibitors which activates the TβR‐V‐mediated tumor suppressor signaling cascade (TβR‐V/IRS‐2/PP1/pRb) to maintain pRb unphosphorylated (active).