u201c biodegradable linker u201d as used herein refers to a nucleic acid or non nucleic acid linker molecule that is designed as a biodegradable linker to connect one molecule to another molecule for example a biologically active molecule to a sina molecule of the invention or the sense and antisense strands of a sina molecule of the invention the biodegradable linker is designed such that its stability can be modulated for a particular purpose such as delivery to a particular tissue or cell type the stability of a nucleic acid based biodegradable linker molecule can be modulated by using various chemistries for example combinations of ribonucleotides deoxyribonucleotides and chemically modified nucleotides such as 2 u2032 o methyl 2 u2032 fluoro 2 u2032 amino 2 u2032 o amino 2 u2032 c allyl 2 u2032 o allyl and other 2 u2032 modified or base modified nucleotides the biodegradable nucleic acid linker molecule can be a dimer trimer tetramer or longer nucleic acid molecule for example an oligonucleotide of about 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 or nucleotides in length or can comprise a single nucleotide with a phosphorus based linkage for example a phosphoramidate or phosphodiester linkage the biodegradable nucleic acid linker molecule can also comprise nucleic acid backbone nucleic acid sugar or nucleic acid base modifications by u201c abasic u201d is meant sugar moieties lacking a base or having other chemical groups.