Possesses an intrinsic kinase activity (PubMed:16313181). In vitro, does not exhibit nucleoside diphosphate kinase (NDPK) activity or disulfide bond-reducing activity (PubMed:11737268, PubMed:16313181). Additionally, exhibits a 3'-5'-DNA exonuclease activity that removes single nucleotides from the 3' terminus of single-stranded DNA substrates and digests overhanging mismatched 3' termini from double- stranded DNA substrates, suggesting a role in DNA nucleolytic processing (PubMed:16313181). May be required during the final stages of sperm tail maturation in the testis and/or epididymis, where extensive disulfide bonding of fibrous sheath (FS) proteins occurs (By similarity). {ECO:0000250|UniProtKB:Q715S9, ECO:0000269|PubMed:11737268, ECO:0000269|PubMed:16313181}. This is the function of ENSG00000086288 (NME8, NME/NM23 family member 8).