Chiron scientists have focused on modifying or eliminating the enzymatic activity of LT (ADP-ribosylation) through single amino-acid substitutions in the active site of A1 (LTK63 and LTR72), while an alternative approach pursued by others was to modify the ability of the A1 subunit to be proteolytically cleaved from A2 by trypsin (LTR192G). allow vaccines to be used to treat chronic infectious diseases, autoimmunity and cancer.