The present invention relates to the expression and screening of genomic DNA sequences encoding uncharacterized genes and proteins. The present invention provides systems utilizing unique features of retroviral replication to analyze uncharacterized genes derived from genomic DNA samples. In preferred embodiments, a segment of genomic DNA is inserted between 5' and 3' viral long terminal repeats (LTRs) in a vector (e.g., a plasmid, cosmid, or artificial chromosome vector). The resulting vector (or library of vectors containing a plurality of independent genomic sequences) is then introduced into a retroviral packaging cell. The resulting provirus or proteins expression from the provirus are then analyzed.