3 can be used to generate construct 2 under the same principle wherein a linker is used to generate the active sirna construct 2 in vivo and or in vitro which can optionally utilize another biodegradable linker to generate the active sirna construct 1 in vivo and or in vitro as such the stability and or activity of the sirna constructs can be modulated based on the design of the sirna construct for use in vivo or in vitro and or in vitro 0141 fig7 is a diagrammatic representation of a scheme utilized in generating an expression cassette to generate sirna hairpin constructs a a dna oligomer is synthesized with a 5 u2032 restriction site r1 sequence followed by a region having sequence identical sense region of sirna to a predetermined hiv target sequence wherein the sense region comprises for example about 19 20 21 or 22 nucleotides n in length which is followed by a loop sequence of defined sequence x comprising for example between about 3 and 10 nucleotides b the synthetic construct is then extended by dna polymerase to generate a hairpin structure having self complementary sequence that will result in a sirna transcript having specificity for an hiv target sequence and having self complementary sense and antisense regions c the construct is heated for example to about 95 u00b0 c to linearize the sequence thus allowing extension of a complementary second dna strand using a primer to the 3 u2032 restriction sequence of the first strand the double stranded dna is then inserted.