The Exonuclease I (ExoI) degrades single-stranded DNA in a 3'=>5' direction, releasing deoxyribonucleoside 5'-monophosphates in a stepwise manner and leaving 5'-terminal dinucleotides intact. It does not cleave DNA strands with terminal 3'-OH groups blocked by phosphoryl or acetyl groups. Source: E. coli cells with a cloned E. coli sbcB gene. Concentration: ~20u/ul Unit Definition: One unit of E9007-85C catalyzes the release of 10 nano-moles of acid soluble nucleotides in 30 min at 37°C. Activity Assay: 67mM glycine-KOH pH 9.5, 6.7mM MgCl2, 1mM DTT, 0.17mg/ml single stranded [3H]-DNA.