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Architecture of Alu, B1, and B2 repeat elements. (a) Alu elements are about 300 nucleotides long composed of two arms joined by a mid. A-stretch and terminated by a poly (A) stretch. They contain two boxes (A and B) of the RNA polymerase III internal promoter. (b) Alu RNA secondary structure (adapted from [23]) and (c) B1 and B2 RNA secondary structure (adapted from [24]).
