HK2 and cancer: Several other novel CPPs for targeting cancer cell mitochondria, including Pal-pHK-pKV, an engineered peptide performed with the N-terminus of the HK-II protein [524]; pHK-PAS, achieved by covalently coupling N-terminal 15 aa of HKII (pHK) to a short, penetration-accelerating sequence (PAS) [525]; MTP3, another engineered peptide synthesized via resin-based solid-phase peptide synthesis, are also serving as efficient tools to deliver exogenous therapeutics into mitochondria and representing promising strategy in cancer therapy.