Abstract: Isolated polynucleotides, and vectors including the same, are disclosed as useful for down-regulation of specific RNA in cells, including a first sequence of about 17 to about 23 nucleotides, complementary to said RNA, and linked to a second sequence capable of forming a loop when said second sequence is RNA. The polynucleotides include self-complementing single-stranded polynucleotides, including a third sequence linked by said second sequence where all nucleotides in said first and said third sequences are complementary. Functional genomic, diagnostic and therapeutic methods are disclosed that involve reducing the amount of a unique RNA sequence in cells using a vector encoding the self-complementing polynucleotide including a first sequence complementary to said RNA sequence. Methods are also disclosed for preparing the polynucleotides, vectors, libraries of vectors, and the temporary knock-down of proteins, such as lethal proteins, during virus or recombinant protein production.