The present invention relates to nucleic acid molecules containing spacers that can be packaged into viral particles and methods of producing them. In a first aspect, the invention features a nucleic acid molecule including a first spacer (SSI) a first inverted terminal repeat (ITR1) a cloning site (CS) a second inverted terminal repeat (ITR2) and a second spacer (SS2), such as a eukaryotic spacer operably linked to each other in a 5-to-3 direction as: SS1-ITR1-CS-ITR2-SS2. In an embodiment, the invention features a vector comprising any of the above-described nucleic acid molecules. In another aspect, the invention features a plurality of viral particles including the nucleic acid molecule. The invention further includes a host cell including any of the above-described vectors.
