Snapshots of the slicing cycle. The figure shows key structures determined [11–13] and the stages of Ago-mediated silencing they most closely represent. Thermus thermophilus Argonaute (TtAgo) is shown in grey, guide DNA in red and target RNA in blue. The designations given for the structures (for example, 'Apo') are used in the main text and in the other figures. The positions covered by the guide or target strands are listed (numbering from the guide 5' end). Protein data bank (PDB) codes for the structures are as follows: 'Apo' - 3DLB, Binary - 3DLH, Mismatched ternary - 3F73, 12 mer ternary - 3HO1, 15 mer ternary - 3HJF, 19 mer ternary - 3HK2. The figure, together with Figures 2 and 3, was produced using Pymol .