Abstract: This invention relates to the screening of nucleic acids. More particularly, the present invention provides a dysfunctional viral genome capable of both expressing libraries of exogenous nucleic acids and selecting the sequences having a predefined characteristic or function within the cell, such as nucleic acids encoding signal peptides, secreted proteins, membrane bound proteins, proteases and drug-resistance proteins. The invention further provides a method and a kit for selecting nucleic acids having a desired feature, wherein production of a viral particle is dependent on insertion of an exogenous nucleic acid having the desired feature into a dysfunctional viral genome or into a viral genome exposed to a substance inhibiting viral packaging function(s).