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The present invention provides polynucleotides comprising an RNA sequence comprising a first stretch of 21 consecutive nucleotides and a second stretch of 21 consecutive nucleotides, complementary to the first stretch of 21 consecutive nucleotides, wherein in first stretch of 21 consecutive nucleotides: in the 5'-> 3'direction, the first nucleotide is an A-nucleotide, the second nucleotide is a C-nucleotide and the last nucleotide is a C-nucleotide; and no stretches of four or more consecutive identical nucleotides are present; the total number of G-and C-nucleotides is between 33-71% of the total number of nucleotides and in the 5'-> 3'direction, consecutive nucleotides 3-21 are homologous to a RNA-molecule. Vectors encoding the polynucleotide of the present inventionare also provided. Furthermore libraries of the polynucleotides and vectors encoding and methods to produce vectors and libraries are given.