Besides X-family DNA polymerases (first of all, Pol β) several other human DNA polymerases from Yand A-families were shown to possess the dRP-lyase activity and could serve as backup polymerases in base excision repair (Pol ι, Rev1, Pol γ and Pol θ). However the exact position of the active sites and the amino acid residues involved in the dRP-lyase activity in Y-and A-family DNA polymerases are not known. Here we carried out functional analysis of fifteen amino acid residues possibly involved in the dRP-lyase activity of human Pol ι. We show that substitutions of residues Q59, K60 and K207 impair the dRP-lyase activity of Pol ι while residues in the HhH motif of the thumb domain are dispensable for this activity. While both K60G and K207A substitutions decrease Schiff-base intermediate formation during dRP group cleavage, the latter substitution also strongly affects the DNA polymerase activity of Pol ι, suggesting that it may impair DNA binding. These data are consistent with an important role of the N-terminal region in the dRP-lyase activity of Pol ι, with possible involvement of residues from the finger domain in the dRP group cleavage.Human DNA polymerase iota (Pol ι) belongs to the Y-family of translesion DNA polymerases and demonstrates very low accuracy of DNA synthesis. The high error rate of Pol ι is a result of the special organization of the active site 1, 2 which is adapted to bypass a variety of DNA lesions 3 , including bulky carcinogenic lesions 4-9 and interstrand DNA cross-links 10 . The DNA polymerase activity of Pol ι is stimulated by Mn 2+ ions 11,12 . In addition to the DNA polymerase activity, human Pol ι has an intrinsic 5′-deoxyribose-5-phosphate lyase activity (dRP-lyase activity) 13,14 . Pol ι carries out efficient DNA synthesis on gapped DNA substrates and in reactions reconstituted with uracil-DNA glycosylase, AP-endonuclease and ligase can repair DNA 13,15 . Moreover, Pol ι is able to complement in vitro the short-patch base excision repair (BER) deficiency of Pol β null cell extracts 16 .