KLK3 and neoplasm: Once arriving in the tumor microenvironment of prostate cancer (PC), the shielding domain would be cleaved off by the tumor-overexpressed PSA, unmasking of the CPP domain to enhance cellular internalization of the conjugated liposomes.502 Another study has reported that the cell-penetrating capacity of TAT could be substantially hindered by attaching alanine-alanine-asparagine (AAN), a substrate of the cysteine protease legumain, to its fourth lysine.