By comparison of the 3D structures of Polb and TdT , a TdTspecific loop ( loop1 ; 383 - 400 amino acids ) , located in the palm subdomain ( see Figure 1A ) , was proposed to be important for the specific properties of TdT ( 2 ) . By inspection of the multiple amino acid sequence alignments of Polb , TdT and Polm ( 2,6,8 ) it is apparent that a homologous region to loop1 is present also in Polm ( 367 - 385 amino acids ; see Figure 1A and B ) . Moreover , tridimensional modelling of Polm on the TdT crystal structure , carried out as described in Materials and Methods , predicts a similar spatial location of the region homologous to loop1 , located in the palm subdomain ( data not shown ) .