(1) PINK1 phosphorylates nearby Ub molecules in existing K48- and K63-linked chains in outer mitochondrial membrane proteins. (2) Parkin is recruited to mitochondria by surface pUb molecules. (3) Parkin multi-ubiquitinates the attached mitochondrial proteins. (4) New Ub molecules are phosphorylated, thus amplifying the signal. (5) Parkin is eventually phosphorylated by PINK1 which increases parkin's ubiquitination activity, primarily through free Ub chain formation. (6) Unrestricted ubiquitination creates the poly-ubiquitin signal required to stimulate mitophagy.