The unit definition uses 0.12 μM (300 μg/ml) lambda HindIII fragments. The high DNA concentration can be used for linker ligation. To promote circle formation, useful in transformation, a lower total DNA concentration should be used. The overall concentration of vector + insert should be between 1-10 μg/ml for efficient ligation. Insert:vector molar ratios between 2 and 6 are optimal for single insertions. Ratios below 2:1 result in lower ligation efficiency. Ratios above 6:1 promote multiple inserts. If you are unsure of your DNA concentrations, perform multiple ligations with varying ratios.