A Weiss unit is based on the T4 DNA ligase catalyzed ATP-PPi exchange reaction as described by Bernard Weiss, Charles Richardson and colleagues (Weiss, B., et. al., (1968) J. Biol. Chem., 243, 4556). One Weiss unit is defined as the amount of enzyme required to convert 1 nmol of 32P-labeled inorganic pyrophosphate into Norit adsorbable material in 20 minutes at 37°C, using specified reaction conditions (Weiss, B., et al., (1968) J. Biol. Chem., 243, 4543). Norit is a type of activated carbon. A Cohesive End Unit is defined as the amount of enzyme required to give 50% ligation of Hind III fragments of lambda DNA (5´ DNA termini concentration of 0.12 µM (300 µg/ml)) in 20 µl of 1X T4 DNA Ligase Buffer in 30 minutes at 16°C.