Disclosed herein are compositions and methods, similar to CRISPR/Cas9 systems, for modifying a predetermined nucleic acid sequence. A programmable nucleoprotein molecular complex which assembles in-vivo in a target cell comprising a) a polypeptide moiety which comprises a functional domain and a linking domain, and b) a specificity conferring nucleic acid (SCNA) which comprises a complementary region and a recognition region capable of attaching to the linking domain. The programmable nucleoprotein molecular complex is capable of interacting with the predetermined target nucleic acid sequence and is capable of specifically modifying and/or editing a target site within the target nucleic acid sequence and/or modifying the function of the target nucleic acid sequence.
