The invention provides an exonuclease or an enzymatically active fragment thereof, said exonuclease having the amino acid sequence of SEQ ID No. 1 or an amino acid sequence which is at least about 50% identical thereto, wherein said exonuclease or enzymatically active fragment thereof (i) is substantially irreversibly inactivated by heating at a temperature of about 55° C. for 10 minutes in a buffer consisting of 10 mM Tris-HCI, pH 8.5 at 25° C., 50 mM KCI and 5 mM MgCI2; (ii) is substantially specific for single stranded DNA; and (iii) has a 3′-5′ exonuclease activity. The invention further provides a method of removing single stranded DNA from a sample, a method of nucleic acid amplification, a method of reverse transcription and a method of nucleic acid sequence analysis in which the exonuclease or enzymatically active fragment thereof is used. The invention still further provides nucleic acids encoding said exonuclease or an enzymatically active fragment thereof and kits or compositions comprising the same.