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.
