The thumb subdomain has four a helices. Two antiparallel ahelices of them, a-H (Asn 255 to Ser 268 ) and a-I (Gln 278 to Thr 286 ), are important for holding the primer/template in position during the translocation in polymerization. The primary sequence (Val 254 to Ala 288 ) in the vicinity of these two a helices has been found to share homology with several other nucleic acid polymerases and has been termed the ''helix clamp'' [5, 8] . Extensive studies have been carried out to shed light on the relationship between the ''helix clamp'' and function of RT. The effects of alanine-scanning mutations in a-H and a-I on polymerase activity, primer/template (P/T) binding, fidelity and enzyme kinetics have been determined. While mutations in a-I do not affect P/T binding or fidelity significantly, several a-H mutants exhibit lower binding affinity, processivity and frameshift fidelity [9] [10] [11] [12] . Previous studies have demonstrated that mutations in these two helices can have significant effect on RNase H activity, minus-strand DNA transfer activity and removal of polypurine track primer [13, 14] .