Mung Bean Nuclease must be used if the goal is to chew back a 5' extension. DNA Polymerase I, Large (Klenow) Fragment will fill in a 5' extension and it will chew back a 3' extension. Mung Bean Nuclease can be used to chew back a 3' extension, but DNA Polymerase I, Large (Klenow) Fragment may be a slightly better choice due to Mung Bean Nuclease's ability to act at nicks and degrade ends as they "breathe". Note: A 3' overhang can not be filled in.