Comparison of the structural differences seen in Family X BRCT domain loop regions. Secondary structural elements of Pol λ BRCT are shown in the background in light gray. (A). Superposition of the loop between β-strand 1 and α-helix 1 in Pol µ (PDB code 2DUN, maroon), TdT (PDB code 2COE, green), and Pol λ (PDB code 2JW5, blue). (B). Superposition of the loop between α-helix 2 and β-strand 4 in Pol µ (PDB code 2DUN, maroon), TdT (PDB code 2COE, green), and Pol λ (PDB code 2JW5, blue).